On Wednesday evenings, staff at London's probate research offices head home soon after 5pm, as the following morning will, as every week, demand an early start. In the middle of the night, the Government publishes a list of those who have died intestate; that is without a will, and without any close relatives. Most will have been hermits or recluses.

Next to each name on the list are the date they died, an address, and a number. This number indicates the size of inheritance the deceased has left to nobody in particular, and the starting gun for a detective game that changes people's lives.

Over the next few days, weeks or months researchers will track down long-lost relatives, etching in blanks in the family tree, and ultimately making a trip to a lucky individual. They will find out that they are entitled to an inheritance, often millions - the fortunate winners of a genealogical lottery for which they never bought a ticket.

The lucrative probate research industry is dominated by just a handful of companies. In Mayfair, the relative newcomer Kin was founded only three years ago by two ambitious college friends. Matt Siddell was bored with banking and had money to burn, while Sasha Buchler had been doing a form of probate research elsewhere and wanted her own company. They joined forces, found a tiny office with an impressive W1 address, and set to work, sharing the crippling Thursday morning starts and detective work. They have since expanded from their shared desk to the entire floor of the office, taking on a large staff as well as a worldwide network of contacts.

Kin has a James Bond approach: staff have been known to make a start at midnight to get the first look at the intestacy lists and have built up a network of “ex-CID cops” across the country who are on call to follow up any leads and start knocking on doors.

The Wednesday I visit the Kin offices, staff trickle out with a palpable sense of foreboding, as tomorrow the race will be on. “The instant we see the list we get on to it,” Siddell says, opening up the most recent one for me. “From here, we tap the deceased's name into the civil death lists published by the Register Office. That gives us the date of birth.” Siddell scurries round to the a wall full of files... “We've got every birth, marriage and death since 1837 on microfiche here.” He quickly thumbs through records, pulls out a sheet of plastic, and slides it into the microfiche machine. Quickly pulling out a record from a matrix of grids, he finds his man. “There we go, mother's maiden name, place of birth... that's going to set the tone for the whole search.”

The two main probate hunting companies are Fraser & Fraser, and Hoopers, which, in a pleasingly apposite twist, stem from the same family and hotly contest the claim to be the country's first probate researcher. Both descend from A.J. Hooper's & Co, which was founded in 1823. “I don't mind really,” chuckles Mike Tringham of Hoopers, before straightening his face... “but we were the first.”

He has been working at Hoopers for 35 years, after being recruited by the family: “It might not be my name on the door, but it's been so long I feel like family!”

A short stroll from the Clerkenwell offices of Hoopers is Fraser & Fraser, which remains a family business, with twins Neil and Andrew Fraser joint chairmen, and their cousin Charles on board as an in-house legal expert.

All three companies earn their money by taking a percentage of the sum awarded to any heirs they track down, and competition is fierce for the larger estates. Acutely aware of the opposition, each company has developed its own means of hunting down potential heirs as quickly as possible.

Fraser & Fraser enjoys the benefits of the family history in the business, meaning it has an incredible library of Kelly's trade directories, ancient editions of Who's Who, and other dusty volumes that have been collected from book fairs around the globe. It also has a formidable bank of intelligent microfiche machines that can propel them through 300 years of births, marriages and deaths at the whiz of a finger.

Hoopers has its own copy of the Family Records Centre's microfilms, UK electoral rolls and telephone listings - as well as its own archives.

Differences in means aside, all three companies agree on one point: the most useful information source is always the neighbour with the twitching curtain: “If you can get hold of a neighbour they will very often provide crucial details.” Neil Fraser explains: “Who came to visit, or a forgotten niece that once turned up.”

Each company employs mobile agents who will do the local ground work once an address for the deceased is found - that means going to local register offices and libraries, knocking on neighbours' doors, chatting up ancient aunts, and seeking out the crucial pieces of the jigsaw puzzle that might lead to a windfall; Neil Fraser even admits (reluctantly) to finding a crucial contact on Facebook. “Ha ha, yes, but not sure I want that printed!”

“Some cases are obviously easier than others,” Siddell admits, showing me a ludicrously complex and spidery family tree that stretches across several pages. Even worse, the family in question is Welsh. “The Welsh are our worst nightmare,” laughs Buchler. “They're all Lewises married to Joneses married to Evanses and Williamses.” “Oh God,” agrees Neil Fraser. “Your heart sinks when you find out that the deceased's parents were called Smith and Jones. It makes the investigation very complicated.”

Worse still is when the family tree leads the researchers to South America, where children take both of their parents' surnames, leading to chains of 25 surnames; Bali, where there are traditionally only four first names in existence; or Mongolia, where 98 per cent of the population chose to take the surname of Genghis Khan's tribe in the 1960s.

The probate researchers also have to adapt to sociological changes and trends; wars, laws, and habits; surprisingly it was only in 1969 that it became legal for an illegitimate child's father's name to be included on the birth certificate.

When I ask if the new digitalisation of information, and the loss of stigma attached to illegitimacy, might eventually diminish the role of the probate researcher, Neil Fraser points out that this is counteracted by the new mobility of the population: “A family of five might be born in England, and each disperse to a different country where it becomes much harder to trace them. A hundred years ago that rarely happened. Now we also have the issue of civil marriages to contend with.”

The introduction of civil marriages presents a new dilemma for the probate researcher - the issue of surnames. I hazard a guess that there were no heir hunters celebrating the first civil union, as they imagined the extra hours and new techniques needed to start tracing generations.

Frequently, it is the information of which the family is unaware, or has kept shrouded in secrecy, that can provide the crucial link to a surviving relative. Indeed, discoveries can be a double-edged sword to some benefactors. “Illegitimacy is pretty difficult,” Siddell admits. “Adoption didn't come in until 1927. We're just coming to the tail end of that period where people were adopted, but not formally, leaving a lot of loose ends.” In one such case, they reached a dead end with one claimant, whose grandparents seemed to share the same maiden name. Investigations revealed that the claimant's father was the product of incest between a brother and sister; a revelation that takes a little of the shine off a £40,000 inheritance.

They also cite the case of an illegitimate, solitary woman who left a considerable estate. Having traced her mother's progeny from a later marriage, Kin's researchers discovered that the deceased was the product of the routine abuse of her mother by her grandfather.

The two benefactors were thus left a large amount of money from a half-sister who died alone, who never knew them, and who was the unloved product of a very unhappy union. Frasers recently closed a case where the birth certificate of the deceased was riddled with inconsistencies and lies - an attempt to disguise the birth of a child born illegitimately to the local vicar.

Disclosing this sort of news to a family member can be awkward at the least, and at worst traumatic. “You encounter every kind of response imaginable,” says Tringham. “Delight, scepticism, fear, grief and horror... I've been chased from a farm by a shotgun-wielding heir!”

More often than not, however, probate researchers are the bearers of very good news. Aside from bestowing financial gifts on unsuspecting relatives, they are often in a position to facilitate reunions, find lost families and, at the least, provide a fascinating and detailed historical backdrop to each family tree: “We have been called ambulance chasers,” Siddell shrugs, “and I don't think that's fair... We have brought together siblings who were split up during wartime evacuations, helped adopted kids to find a biological family they didn't know they had, and of course telling people that they are entitled to a big sum of money is great. You change a lot of lives.”

A recent case saw them awarding a huge sum of money to a destitute caretaker in a Watford hospital who was unaware that his father had a son from a previous liaison. His unknown half-brother had left him a rich man.

Pitfalls aside, all agree that they share an incredible profession. Aside from the evident material benefits (both Matt Siddell and Neil Fraser confess to a penchant for “silly cars” - ie, ludicrously expensive ones), Tringham says that “the thrill of the chase is still every bit as exciting - every bit - after a lifetime in the profession.”

Perhaps best of all is what Charles Fraser speaks of as the “Robin Hood” element: the taking of free money and giving it to the poor or unsuspecting. As I leave Tringham's office I put in a small request that he keeps a watch out for my own surname in the gold-laden family trees.

With the air of someone who has heard that line many times before, he responds with a sigh: “I'm afraid I have spent 35 years looking for my own name. Sadly, it's the one that never comes up.”

Raising the dead

Bringing back family members from the dead can often be the task of probate researchers, as Mike Tringham of Hoopers found: “A woman had been alone in the family home when it was blitzed, and none of her family realised she had survived. When I visited her brother [living on the streets in Central London, selling the Big Issue], I showed him an old photo and he said ‘Poor Lily, they got her when they got Coventry Cathedral'... I then had to tell him that she'd been alive all that time - he just burst into tears. Fifty years they thought she was dead. You have to be very tactful - I had to check with her that they were happy to meet, but they did, and had an incredible reunion.”

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