Before I start, I must tell you that there is something about this church that annoys me a great deal. If you would prefer not to witness me getting annoyed by Mildenhall parish church, then look away now.

Still here? Alright then. Don't say you weren't warned. But before we come to the church, some demographics.

Mildenhall is Suffolk's biggest parish, covering almost 2% of its entire surface area. Not a lot of people know that. It includes at least five distinct settlements, the largest of which proclaims itself 'Mildenhall High Town'. Less than a third of the population of the parish live in the town, so it is an important reminder to family history hunters in particular to remember the difference between a parish and its town or village. The town centre is tiny, and might once have been pretty. Unfortunately, half a century of occupation by the American Air Force have overwhelmed the parish and worn the town down, making it a shabby, dirty place. It always depresses me.

St Mary is in the heart of the old town, and to the north of the town centre is a large new estate, where we will find the Catholic church of St John. But the character of the Parish is entirely formed by its main employer. Most people who live here work on the vast airbase to the east, part of the Bush empire's main gateway to Europe. The other bit, the base at Lakenheath, is just three miles to the north.

I would find much to say about the American presence during the course of the afternoon I spent exploring the area's churches. But for now, here in the centre of Mildenhall, I was conscious only of the immenseness of St Mary as I stood with my bike beneath the west tower.

It is one of Suffolk's biggest churches, almost 60m long and 20m wide, with a tower 40m high. It dwarfs the majority, and matches the likes of Lavenham and Long Melford. Since the demise of Bury Abbey it is also one of the county's largest medieval buildings, and virtually all of it is original, hardly anything of it Victorian extensions. It is a church of superlatives; the 14th century west window is considered one of England's best, the roof the finest in Suffolk. It contains Britain's biggest Royal Arms, and you enter through one of the biggest church porches in England.

Or rather, you don't. Because this mighty flagship of the body of Christ on Earth is always locked. It is the only urban church in Suffolk that is not regularly open as an act of witness. It is the most significant historic building in the county that you cannot easily walk into. And it gets worse.

I was here on September 13th 2003. I had been several times before, but I had assumed that I would be able to take a look around today because it was the day of the Suffolk Historic Churches Bike Ride. Well, I was in Suffolk, and this Church was certainly Historic. And I was Riding my Bike. What a perfect collision of circumstances! But, incredibly, the church was still locked. There were people on duty, and they were signing cyclists' sponsor forms - but this was happening in the former mortuary across the graveyard. To be fair to them, they were a group of very welcoming and enthusiastic bespectacled teenage girls (I usually find this is the sign of an evangelical congregation), but when I asked if I could go into the church, they looked at me in complete bewilderment. One went off to find her Mum, who was very nice, but told me that they didn't have a key. If I wanted to get in, I would have to go and get the key from a shoe shop in the High Street.

She kindly offered to come with me, but we ended up going the wrong way because she didn't know where the shoe shop was. Note, then, that a regular member of the congregation, committed enough to take part in the welcomers' rota, had not been empowered to enter her own church building easily.

After an extensive tour of Mildenhall town centre, we eventually found the shoe shop. I went in, and waited and waited while the fractious children of two separate families tried on every pair of shoes in the shop. Eventually, one of the tired assistants was free, and noticed I was waiting. I explained my mission. She got the key, and was happy to let me have it, but I would have to pay her five pounds deposit for it.

While this seemed preferable to them asking for formal means of identification like they do at Oulton, it also seemed entirely pointless. If you keep a church locked, it must be because you are intent on protecting your property. Let's just imagine that my main intention for visiting the church was to steal part of what is Suffolk's finest collection of 20th century benches (currently very saleable on the Californian antiques market, or so I'm told). Would having to pay five pounds for the privilege dissuade me? I hardly think so. Once again, we find a security system that seems designed to put off genuine visitors, but not deter thieves.

There's worse to come. Attached to the key is a very large chunk of wood. On the back of it, there is a warning. It says things like Once you are inside the church, lock the door behind you. It says Do not allow anybody else to enter the church. It says While you hold the key you are responsible for the church.

What a load of rubbish. I wonder if the Parish's insurance company knows that the parish is devolving itself of all responsibility in return for a five pound deposit? Let me reassure anybody who is reading this - if you get the key for this church, you are NOT responsible for its contents. They are. In addition, leaving the gates open while you are in there, and letting other people in to have a look around, would be a most Christian act.

Unfortunately, I can't recommend you to do this, because this church seems to have virtually no security systems in place. There are no surveillance cameras, no alarms. These things are so cheap nowadays that there is no excuse for such an important building not having them. I assume that everything is alpha-coded, but I don't know, because there were none of the usual alpha-dot hologrammed stickers about to tell me so - or, more to the point, to tell potential thieves so. This great building is simply not geared up to receiving casual visitors. I would go further. The parish appears not to want them.

I have no doubt that soon after this appears I will be deluged with e-mails telling me that Mildenhall parish church is actually a very welcoming place, but they have to keep the church locked because of thefts and vandalism. I'm sure that it is quite possible that the crime rate is much higher in Mildenhall than it is in Ipswich, where the majority of town centre churches are kept open. Looking around Mildenhall town centre, it wouldn't surprise me in the least. But let's get things in perspective. As Churchwatch reminds us, a church that is kept permanently locked is far more likely to be vandalised than one which is regularly open. It is more likely to be broken into, and is even more likely to have something stolen from it. Most insurance claims received by Ecclesiastical Insurance are made for incidents that occured at locked churches.

Perhaps the day will come when the parish sees the light, and we will all be able to visit this great building whenever we want to. When that glorious day comes, what will we find?

The outside first. The fact that this magnificent pile is in the middle of a scruffy little town only accentuates its glory. But perhaps it is also that which makes it seem curiously aloof. What you see is largely a rebuild of the mid-15th century, in common with the other great Suffolk churches. What makes Mildenhall special is that the Decorated chancel survives from an earlier building, its flowery east window a reminder of how mystical we all were before the Black Death turned us serious. However, it is deficient of the chancel aisles we find elsewhere in churches of this size.

The tower was completed in the 1460s, apart from the stair turret which rises above the battlements, an unfortunate Victorian addition. It works on the trim towers of the Stour Valley, but not here. If you look up at the tower as darkness falls you will see that there is an aircraft warning beacon up there, a necessary feature on this, the only high building in the parish.

Beside the south side stands the medieval charnel house. This was a necessary feature of any medieval town church with a small graveyard; after thirty years or so, corpses would be exhumed and placed in the underground vault, to make room for more burials in the churchyard. The charnel house had its own chantry priest, who said Masses for the souls of the dead. Charnel houses, along chantry priests, fell into disuse after the Reformation, when there was no longer a theological imperative for their use. In fact, the remains of this one were 'gothicised' in the 19th century to make a picturesque ruin. Another one survives in Bury St Edmunds from the lost church of St Margaret.

The south porch is pretty much Victorian, but in any case it is the north porch which is outstanding. Many Suffolk porches have an upper room, but none are as commodious as this one. It formed a chapel to the Blessed Virgin before the Reformation and was later the town school. The vaulted ceiling of the porchway is of a size and quality seen rarely elsewhere in Suffolk. It is bigger than many transepts. Indeed, I have been in smaller churches than this porch.

The south porch should not distract you from the fine west doorway. You can't help thinking that it was intended to be used as the main entrance. Once you are inside, however, all thoughts of the outside will driven from you by the sheer vastness of the interior. It is tempting to turn your attention immediately eastwards or upwards, but if you turn west and go through the curtains into the space beneath the gallery you will discover a wonderful vaulted ceiling here as well. The two lancet glasses here are probably the best kept secret of the building. They are beautiful, I think.

In the gallery above hangs what is claimed to be the largest Royal Arms in England. It is certainly very big, and must have created quite an impression when it was suspended above the chancel arch.

Today, the eastward view is complemented by the banners that hang from every pillar. These are one of the better inventions of the 20th century Church, and are to be encouraged, I think. They must be a useful distraction when sermons are dull, allowing a little theology still to seep into the daydreaming soul. It just goes to show that you can't escape the Word. The one in the chancel arch says Holy Spirit we welcome you. I wondered if that meant He doesn't have to pay a five pound deposit to get in. My favourite here is actually hanging on the tower gallery - it says Patience, Kindness, Self-control. It immediately caught my eye as I flounced into the building in a bit of a temper after the episode of 'getting the key'. Oops, sorry God, I whispered.

I couldn't help thinking that a community so enthusiastic about banners, particularly one that suspends one from the rood screen, might not be entirely comfortable with the medieval nature of its building.

You come to Mildenhall to see the roofs. The most spectacular is the nave roof, but the two aisles are even more interesting and rewarding of time and binoculars. Mildenhall's nave angels are restored, although not as heavily as those at Woolpit. Somone was paid a shilling a day to destroy them in the 1650s, but they have suffered less from iconoclasm than from 18th century churchwardens firing into the air to disperse jackdaws. They are not beautiful like those at Blythburgh, but they are certainly magnificent.

But the aisle roofs are the best in England. The north aisle is the better of the two.The massive hammerbeams are carved with the most extraordinary figures, and you can't help thinking that they don't need to be so big. You're right - in fact, they don't need to be there at all. Both aisle roofs are supported by their own weight on the arcades, and all the hammer beams are false. It did occur to me to wonder for a moment if they had been made originally for a quite different church, and used here instead. Most of the hammer beams feature prone figures with grotesque faces, all of them rather startling. Note the one third along from the east; it has the blade of a pike embedded in it, presumably as a result of that vile puritan trying to bring it down.

The beams may distract you from the carvings on the spandrels, which are exquisite. My favourites are the Annunciation, the adoration of the shepherds, and St Michael with his dragon.

All the furnishings are modern, and were given by Munro Cautley in memory of his wife. He designed them, and to be honest they aren't bad. Like a lot of his work, they have a flavour of 1930s cinema architecture about them, despite dating from 1959, but the carved figures on the bench ends at the far west are super, with St Etheldreda of Ely on one side, and the Blessed Virgin on the other.

The font isn't strikingly beautiful, but it does include the arms of Sir Henry Barton, Lord Mayor of London, whose tomb is at the west end of the south aisle. Mortlock thought he might have been the major donor for the rebuilding of the church. Incidentally, it is worth recording that Mr Mortlock was a choirboy here in the 1930s, and his enthusiasm for the building shines through.

There are a couple of significant memorials, one of which is at the eastern end of the south aisle. This is early 17th century, and commemorates Sir Henry North, Lord of the Manor. I was disappointed to discover from Mortlock that the weeping figures are 19th century replacements. The Norths were still around after the Commonwealth, and not in a terribly good mood; Sarah North, wife of Henry Jr, has a memorial beside the chancel arch on which her husband records in Latin that he is Dead while living, oh how hard; you are happy because your life has ended, I am desolate because I cannot die. He wrote the gloomy sequence Eroclea or the Mayd of Honour, and then promptly killed himself. Puts a bit of a downer on the whole place, don't you think?

The rood screen is a bit of a disappointment. The fact that there are two upper doorways to the original rood loft stairs suggests that it must have been a massive affair. The screen you see today is a 20th century replacement. Note here the memorial to Cautley and his wife opposite the North memorials. They are also commemorated (and buried) at Westerfield, across the county.

It is easier to be grand in Perpendicular than it is in Decorated. Because of this, I think that the chancel is really rather splendid, and often overlooked except for the window. Mortlock calls our attention to a graveslab in Lombardic script in the sanctuary. It remembers Ricardus de Wichforde who made this new work. He it is we have to thank for the wonderful window tracery. To imagine it with its original glass is to recall and regret how much we have lost.

So, there it is. I spent a happy (and obviously very quiet) half hour documenting it all. Then I let myself out, and, carefully leaving the gate unlocked behind me, returned the key to the shoe shop. Once again, I waited to be served while two irritable parents snarled, their children wallowed in a vale of tears and frustration, and shoeboxes piled up around the now-exhausted assistants. Eventually my five pound note was returned to me (can I suggest that, when you visit, you pay this deposit with five hundred 1p coins? It might make them think twice about asking for it in future) and went on my sardonic and slightly embittered way.

St Mary, Mildenhall, is right in the middle of the old town. For reasons that I hope are obvious, you can't miss it. The key is at the shoe shop facing the market cross.

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