At the age of 16 he became a researcher for the College of Arms, moved on to Phillips the auctioneers at 20, and then joined Christie's as a consultant on coats-of-arms.
Now, aged 30, he is a freelance genealogist in London, delving into other people's family trees. But he is still fascinated by his own, and it was while he was chasing up the 'pedigree' of his grandmother's Dutch-colonial ancestry last month, that he stumbled upon some information which would lead to his most extraordinary discovery.
He chanced upon a back copy of Family Tree (a genealogy journal) from two years ago, which reported that it had established that John Major and Margaret Thatcher both came from families with the same name - Crust - living in the same town - Boston in Lincolnshire - in the middle of the 19th century.
By odd coincidence, both politicians were descended from men named Samuel Crust. One Samuel Crust was a 'fettler' in an iron works, whose job it was to line the furnace with loose rocks; the other was a corn-porter. Although the information indicated that they were different people, the intriguing question was: were they related?
The Family Tree article did not say whether there was any common ancestor between the two Crust families. But if one existed, Mr Penn-Simkins realised, it could mean that Mr Major and Baroness Thatcher were cousins.
Inspired by this remarkable possibility, he headed for the public record office in Chancery Lane.
There he looked through the census returns of 1871 and established that Samuel Crust the corn-porter was married to a woman named Lucy, and had a daughter, Lucy-Ann: this was John Major's ancestor. Samuel Crust the fettler - Lady Thatcher's forebear - lived with his wife, Eliza, and their daughter, Phoebe.
Both these girls married almost within a year of each other (in 1875 and 1876) at the same church: St Botolph's in Boston.
Census returns can be unreliable, however: 'Sometimes householders didn't answer truthfully,' says Mr Penn- Simkins. 'For instance, wives may not have liked their husbands to know their real ages, and husbands upgraded their jobs.'
The parish records of St Botolph's are held in Lincoln Archives, so Mr Penn-Simkins went to have a look. He discovered that although Crust was a fairly common name, meaning simply 'stubborn and hard' people, those who came from Boston could be numbered on the fingers of one hand.
Looking at the parish registers of Boston and the surrounding Fens, he saw that other members of the Crust family had laboured variously as 'cottagers', 'graziers', 'watermen' and 'carriers': jobs of similar status to those of both the Samuel Crusts. It was a promising sign. So he decided to go on to Boston to look for more clues.
BOSTON is tucked into a corner of the Wash. Its chief glory is 'Boston Stump', the truncated tower of its medieval church. During the 17th century it was one of the most important centres for Nonconformists: John Cotton, vicar of Boston, led the exodus of Puritans from the town to America in 1633, and his legacy lives on in the form of 'Pilgrim' tearooms and 'Pilgrim Frozen Foods'.
Since then Boston has boasted notable inhabitants such as Sir Joseph Banks, explorer and botanist, and Herbert Ingram, founder of the Illustrated London News. Ships still adorn the port, which during the 1300s rivalled London in importance.
In the 19th century - the time that John Major's corn-portering ancestor was at work - Boston was at the height of its success as an agricultural centre, as the magnificent abandoned corn warehouses situated round the town testify.
Nowadays Boston is a centre for canned fruit and veg, with a population of 27,000. Cabbages and turnips line huge tracts of the relentlessly flat land that surrounds the town, which until recently was called the parish of 'Holland'. It is not a poor town: although 9 per cent of Bostonians are unemployed, it has done better than many places in the recession. But it is a quiet town: J B Priestley felt it was the 'remotest' part of England.
Mr Penn-Simkins' first port of call was St Botolph's. To his disappointment, most of the tombstones in the churchyard had been removed, and the church doors were bolted. So he went on to the cemetery on the outskirts of town where he knew that one of the Samuel Crusts had been buried. The cemetery keeper, a Mr Knowles, helped him to find the place where the grave had been. But, frustratingly, 'nothing was there - not even a headstone. Nothing except clumps of wild garlic'.
Mr Penn-Simkins went back into the town to try to seek out the one-time homes of the two Samuel Crusts. The first one he visited was that occupied by Lady Thatcher's iron-fettling ancestor and his family. 'It was in Victoria Place, a fairly nondescript terrace of houses set in the shadow of the old Vulcan ironworks where the head of the household worked. It's a beige pebbledash house with an enormous satellite dish. I tapped on the door, but there wasn't even a tweaking of the lace curtains.'
John Major's forebears, he discovered, had lived barely two minutes' walk away, in Duckfield Lane. 'Tragically, all that I found where the house once was was the decaying shell of a 1970s warehouse - the Boston Bedding Centre.'
He turned to the phone book as the last resort. Expecting to be confronted with columns of local Crusts, he was astonished to find that in Boston there were only two. The first he rang was G A Crust - who professed no knowledge of any relationship with the Tory leaders at all. The other entry was for I Crust, of Tower Street, Boston.
A Mr Geoffrey Crust answered the phone. In an impatient Lincolnshire accent he told Mr Penn-Simkins that 'yup' - he was aware of the link that had been made with his family and both Tory leaders. He had been contacted by his brother John - a policeman in Gainsborough - who had already done some research into the family tree with the help of a researcher at the Lincolnshire Archives, Lynda Rippin. He himself was a cook in the Merchant Navy. Mr Penn-Simkins asked if he could go round and see him. 'We can talk about it a bit if you like,' was the reply.
'He ushered me into his sitting-room, and with a flourish produced a huge handwritten pedigree,' says Mr Penn-Simkins.
'Scrawled out in black felt-tip over a piece of paper about 4ft square were the labyrinthine workings of the Crust family dating back to the 1600s.
'There, laid out on his dining table, were the answers to all my questions. This man, I realised, provided the living link between John Major and Lady Thatcher. He was John Major's sixth cousin and Lady Thatcher's fourth cousin once removed.
'The astonishing fact which this piece of paper revealed was that John Major and Margaret Thatcher are fifth cousins once removed. (See family tree, right.) Both are descended from a couple called John and Elizabeth Crust who had farmed in Leake, a village six miles from Boston, in the mid-18th century. And Geoffrey did, too.' (The troublesome Samuel Crusts were in fact second cousins.)
Geoffrey Crust had done what nobody else had managed to do - pieced together his connection with Lady Thatcher, and from there deduced that both she and he were related to Mr Major. He had kept this family tree at home.
Asked if he had shared his discoveries with cousins Margaret and John, he modestly confided that he was 'hoping to get all this together and send it off to them'.
Mr Penn-Simkins was charmed by Geoffrey Crust. 'He seems to have all the qualities you would associate with an old seadog, with a really genuine character. It's rather sad that John Major and Margaret Thatcher haven't met him because he is the type of man you would like to have on your side. For me, it was amazing to see a pedigree living and breathing. Dry research work doesn't often provide that]'
A phone call to Lynda Rippin at Lincoln's archives office confirmed Geoffrey Crust's pedigree. Ms Rippin, a genealogist employed by Lincolnshire council to help members of the public trace their ancestry, said she had been helping Mr Crust construct his family tree for the past couple of years.
Like Mr Penn-Simkins, she had been intrigued by the article in Family Tree, and had begun some work of her own on the connections between the Crusts, Mr Major and Lady Thatcher. She had consulted the local records in Lincolnshire Archives such as parish registers, settlement certificates and wills. However, she had come to a dead end in her search for the common ancestors. When she was contacted by Geoffrey Crust, it was the information about his branch of the family that provided the missing pieces of the jigsaw.
By pooling their knowledge, Ms Rippin and Geoffrey Crust were able to piece the family tree back to 1695 - to one couple, Charles Crust, a labourer of West Keal, and his wife, Elizabeth. Ms Rippin checked all the records, and drew up the detailed family tree (shown right). Her original copies are held in Lincolnshire Archives.
Having tracked down the biggest genealogy scoop of his career, Mr Penn-Simkins has now been commissioned to write a book on genealogy and heraldry for children, and is looking for new families to trace.
Geoffrey Crust is off to sea for a while. But he hopes that one day he might be invited to No 10 to make the acquaintance of his sixth cousin, and his fourth cousin once removed.
(Photographs, graphic and map omitted)