`Mr Lilly," said Oliver Cromwell, "I desire you would use all your skill to paint my picture truly like me, and not flatter me at all but remark all the roughness, pimples, warts and everything as you see me. Otherwise, I will not pay a farthing for it."

And there in the resultant portrait - one of the items in the London Museum's exhibition Oliver Cromwell Warts and All, set up to mark the 400th anniversary of his birth - we see the gleam on the Lord Protector's armour, the white linen collar, the long hair beginning to recede, the slight frown and the large wart, just below the bottom lip. Except that, as in many things to do with Cromwell, all is not what it seems. The painting is a copy: the original, by Sir Peter Lely, is in the Pitti Gallery in Florence. No one is certain that he actually made the (misquoted) warts and all comment, and the world is still out on whether he was the devil incarnate, a deluded fanatic or a Godfearing statesman.

Remembered - or misremembered - mainly as the man responsible for the beheading of Charles II (although there were 58 other signatories to the death warrant), for stabling his horses in every Catholic church in the land, for persecuting Catholics generally and Irish Catholics particularly, for doing his best to take all the fun out of life, there have been enough things in Cromwell's favour to ensure he is still a controversial figure.

More has been written about him than any other British ruler (4,000 books at the last count), his remarks have been repeated and recycled in every century since his death from malaria in 1658 and his trail of relics is miraculous in that many of them crop up in places which he had never visited. The London Museum houses the largest collection of Cromwell memorabilia in Britain; the current exhibition is only the tip of the iceberg, but it is an intriguing one. Included is a button from a shirt reported to have been worn by him when he "sett to judge King Charles 11"; an elegantly carved ebony ale tankard, depicting Cromwell dissolving the Long Parliament, his hand hovering over the famous bauble; a gilded silk funeral escutcheon taken from the hearse that bore his body to Westminster Abbey; his signature - firm and legible - on a military order; the shirt worn by Charles II at his execution; a great number of paintings and statuettes, for there were many likenesses made of him in the 18th and 19th centuries, evidence that his character continued - and continues - to engage. Small wonder, for here is a man who, over a period of some 11 years, travelled from relative obscurity to become a brilliant soldier and head of state, ruling over three kingdoms without being crowned, the only British ruler ever to do this. During his time, he challenged the raison d'etre of the monarchy - the divine right of kings - and set about establishing a parliamentary system which laid the foundations for Britain's present democratic system - one good reason why his statue holds pride of place outside the House of Commons. Having subdued Ireland - the massacres at Drogheda and New Ross go unremarked in the exhibition - he returned to the Parliamentary stronghold of London, where he received a tumultuous welcome.

Hazel Forsyth, curator of the exhibition, set out to present the side of Cromwell usually concealed by the warts. He was not, she says, a dull, dreary Puritan. "He loved music. There was dancing at his Whitehall court and ale houses were not closed. They were strictly licensed in an attempt to curb the drunkenness prevalent at the time. The theatres were closed, true, but this was partly because of the plague and, in any case, the play form continued in pageants. And I didn't refer to the massacre at Drogheda because I was limited to 100 words per exhibit note and it would have needed a great deal more than that."

Fundamental puritanism was only a small aspect of the Cromwellian era, and cultural life continued. Education and learning were particularly valued, and Cromwell made significant improvements to Oxford's Bodleian Library. Under his rule, the first English opera was staged in London, The Siege Of Rhodes, written by Sir William Davenport and lavishly designed by John Webb. Milton, Andrew Marvell and John Dryden ensured that the poetry of the time would never be forgotten. Cromwell surrounded himself with paintings and had many self-portraits commissioned. (The exhibition includes a portrait of Elizabeth Cromwell, to whom he was happily married and who bore him eight children.)

There were few great buildings designed during the period, since, after seven years of Civil War, when so many buildings were destroyed by both sides, landowners and patrons were reluctant to embark on any ambitious schemes. There were exceptions, however, such as Belvoir Castle, and Thorpe Hall, near Peterborough - which suggest a move away from Italianate style towards a restrained northern European classicism - and Holy Trinity Church, in Leicestershire, designed in a nostalgic Gothic style. Cromwell's aims were to create a country in which godly behaviour might be pursued, and where Protestant groups might practice their faith undisturbed. Royalists and their Catholic supporters were regarded by him as the main threat to this last aspiration.

When he was buried, the funeral hearse cost £4,000 (the average wage then was one shilling per day) and the route was decked with 2,000 banners. In the course of embalming, his skull was cut open and the brain removed - but when, 10 years later, the people decided to return their monarch to the throne, his body was exhumed and hanged at Tyburn, now Marble Arch. Later, his head was severed and placed on a pike outside Westminster Hall. In 1960 a head, almost certainly Cromwell's, was taken to his old Cambridge college, Sydney Sussex, where it remains today, its precise location known only to a few people, including the Master and the Chaplin. A bizarre painting in the exhibition shows the head resewn to the body, the stitches clearly visible, with three female figures wailing in the background, one of them representing Ireland.

There are plenty of streets in Britain named after Cromwell - but equally, there were some who did not want to be tainted with his name. Less than 40 years ago, an Oxfordshire borough blocked a proposal to name a new housing estate after him: "We have more than enough benefactors whose names we would like to commemorate without entertaining a malefactor in his class," a councillor told the local paper. January 30th, this year, will be the 350th anniversary of the execution of Charles I, and, during the Civil War, Oxford was a Royalist stronghold. Who says it's only the Irish who never forget?

Cromwell: Warts and All is at the London Museum until February 28th 1999. Open Monday to Saturday 10.00-17.50. Sundays 12.00-17.50. Admission: a year's ticket to the whole museum costs £5 for adults and £3 for children.

There are a number of events commemorating the 400th anniversary of Cromwell's birth taking place throughout Britain, including re-enactments of Civil War battles. Details of these, and of the Museum exhibition will be found on: www.cromwell.argonet.co.uk