Mary Clow can trace her ancestry back to a mistress of Charles II. Here she describes the day when she signed a release form against possible infection, and entered the family vault, a charnel room where five centuries of her progenitors lay sealed in lead awaiting Judgement Day. Photographs by Christopher Sykes
Some waggish genealogist has calculated that everyone claiming to be English, duke or dustman, descends from a single couple alive in the 14th century. Considering how the English dread spending time with their families, this prolific pair must be pitied at the Last Trump, when hell is other people - all of them relations. Probably the only significant family difference between the classes is that while the dustman remembers his dear old Nan and a few names beyond her, the duke eats his dinners under his ancestors' faces and can look them up when he's forgotten who they are.

But if we English plod through centuries of shared genetic porridge, few people get to paddle in it literally. Last month, I put on protective overalls and, having signed a release to take full responsibility for any infection incurred, was invited to climb through a gaping hole in the floor of a village church and descend a flight of stairs. There, in a vault well-aired by a secret vent within the thickness of the wall, five centuries of my progenitors lay awaiting Judgement Day: Norths, sealed in lead to prevent putrefaction, invisibly preserved in the ruffs and velvets of their living finery - for them no dust to dust with the parish in the graveyard outside. The proximity induced questions: who were these dead who thought their preservation as important as any pharaoh's?

Edward North was a lawyer in the Tudors' dream-team, so useful to the dynasty that he served Henry VIII, his young son, Edward VI, the doomed Lady Jane Grey, Bloody Mary, and survived to die in his bed under Queen Bess. For Henry he dispersed the riches of the Church, for Mary he suppressed heretics - and profited from both persuasions. Mary made him Baron North of Kirtling, where he demolished a pre-Conquest castle and used bricks from the new kilns at Ely to build a stately home for his growing family.

His youngest son was Thomas, an inspired translator from French, Italian and Spanish, whose Plutarch's Lives was the 16th century bestseller that Shakespeare freely plagiarised - it gave him six good plots.

Edward's monument in Kirtling church is as plain as a Puritan's, listing his attributes in the skilful Latin that had secured his fortune. Next to him, his heir, Roger, 2nd Baron, lies in Renaissance splendour, his life-size effigy in full armour under the canopy of a six-poster tomb. As a teenager, Roger had jousted wearing the Princess Elizabeth's red silk scarf on his arm. She never forgot the friendship, and, on her accession, the first house she stayed at belonged to the Norths. The custom of providing royal hospitality continued towards the Stuart kings, but as horse racing became established at nearby Newmarket, the Lord North of the day had to decamp to London to escape his guests' ruinous visits.

There are more intimate ways a subject may serve his king. Meeting a favourite cousin at a family wedding I blurted out: "Do you realise we're descended from two of the mistresses of Charles II?"

"My lot claim we're descended from all six of his mistresses." my cousin replied.

"You were ennobled for that?"

"No. Catering. Gave a good party." (In 1799, the Lord Lt of Kent gave a dinner for George III and 5,319 Kentish Volunteers. He was created Earl of Romney.)

It can get yet more personal. Francis, 7th Baron North, was Lord of the Bedchamber to Frederick Prince of Wales, George II's heir, who died before becoming king. Francis, and his young wife, prospered at Court and later he was created Earl of Guilford. Was it a courtier's sycophancy to name their son after Prince Frederick? Was it a chance that Frederick North's physical resemblance to the Prince's son, George - even to their laziness, obesity and blindness - was startling? For whatever reason, George III made Frederick North his long-time trusty Prime Minister - with liberating results for the American colonists. Six generations later, police patrols regularly picked up a North aunt, scantily clad, talking to trees. Alas, no one thought to check her chamber pot or send for Alan Bennett.

And so on a windblown winter morning, a group of cousins converged on a Norman church hunkered down in the flat country east of Newmarket. Largely through the terrier-tenacity of Major Tony Tavener, vice-chairman of the Parish Church Council and Restoration sub-Committee, an 80 per cent grant towards an estimated pounds 130,000 had been approved by English Heritage for the restoration of the roof, tower and fabric of Kirtling church and the conservation of the monuments of the 1st and 2nd Barons North. Ecclesiastical architects and archaeologists needed to check the structure of the vault underneath before any work began, and family permission was asked for North dead to be disturbed. The vault had last been opened 30 years before, for the reception of the Lady North - she was 98, so mortality was something of a family rarity. The location of the entry-stone had been lost, but the practical skills of the conservation team found it by deduction and experience, and Major Tavener telephoned us to say the day was set for lift-off.

As at any English social event, we arrived feeling insecure, but were soon emboldened by the ridiculous appearance of everyone else. Major Tavener had provided stunning white PVC protective jump-suits which those planning to enter the vault were encouraged to wear - the dozen cousins and conservation team alike. The exact nature of the risk was undefined (and later, one determinedly investigative cousin was seen dipping her finger into some catafalque top dust and reflectively tasting it). Just as the reunion began to deteriorate into a funeral party with no corpse, the Rev Christine Sindall, Vicar of Kirtling, called us to order. In her Advent-green hard- hat and overalls neatly zipped to show her dog-collar, she directed us to the family pew (set at right-angles to the congregation the better to observe the parish). We waddled in and bowed our heads like penitent Teletubbies. "This is a happy family meeting," said the Rev Christine, "but we are about to enter a grave." Chastened, we silently asked to see nothing untoward, as the Rev Christine led us in prayer. Then, two at a time, we were invited into the vault.

The raised flagstone in the floor of the north chapel revealed a narrow opening, not easy to negotiate in a PVC jumpsuit and perilous to manoeuvre with a lead coffin. A flight of about 10 red-brick steps vanished underground, lit by a temporary halogen light rigged up by the conservation team. Brightly exposed as never before in its long history, the vault seemed innocent of death, eternal cold-storage which of course is exactly what it was meant to be. It took time to accept that the strange mummy-like shapes - narrow at the foot, widening over hips and shoulders, rounding for the head - held human bodies, wrapped in ancient lead. They were laid in a tidy row like chickens at Tesco's, while the vault itself with its home- baked bricks from floor to ceiling, pure air, order and neatness, resembled a modest cellar where the faithful butler would turn the claret in case a cork dried out. And then, in the far back corner, one suddenly came across what the ecclesiastical archaeologist discreetly referred to as "charnel" - a mess of bone and skulls, jumbled together against the wall of the vault in an untidy pile. The skulls were an exaggerated rich amber, as in the Magdalens of Georges de la Tour. Their teeth had rotted loose, the eye sockets were dark, and the carelessly stacked long bones had once been thighs of forebears who loved and rode and bowed to honour the king. Two by two, we climbed back gratefully into the clear radiance of this very Church of England place of worship - an ambience of calm reasonableness, as devoid of dread as a public swimming pool. Everyone was suddenly quiet, wandering off alone to study some feature of the church away from the others.

There is not much room left in the tomb, and there won't be many takers from today's family.

The Rev Christine is at my elbow. "Have you seen our Green Man?"she asks, pointing at the roof. I make out a grimacing head, eyes bulging, hair and beard formed of leaves and a huge rude tongue stuck out in a gesture of gloriously pagan insolence. "Ely Cathedral is full of them," she says cheerfully. "People like to cover all possibilities."

I walk back to the severe black and white table tomb, with its immaculate Latin redolent of the Reformation's new learning. There, below the lid bearing the inscription, lurks the tousled leaf-haired head of the Green Man. Old Edward North covered all possibilities