This year, 7 November marks the 20th anniversary of the brutal murder of Sandra Rivett, nanny to Lucan's estranged wife, the attack on Lady Lucan the same night, and the disappearance of the earl.
The killing took place in the Lucan family house in Belgravia where Lady Lucan lived with the couple's three young children. Mrs Rivett, 29, was bludgeoned to death after going to the basement to make tea; when Lady Lucan came to find her, she too was beaten about the head - she claimed by her husband. Lord Lucan, who was in the house, fled, and vanished the following day.
No one, he told his friend Susan Maxwell-Scott that night, would believe the truth: that he happened to be walking past when he saw a fight taking place inside. That he had let himself in, disturbed a male intruder, slipped in a pool of blood and had been accused by Lady Lucan of hiring a hitman when he went to her aid.
Pronounced guilty in his absence at Mrs Rivett's inquest, Lucan, then 40, became the first British peer in more than 200 years to be wanted for murder.
The case is being rehashed in no fewer than three documentaries and three books this year. Like most previous accounts, almost all will start from the premise that Lucan is guilty even though he denied it and never stood trial.
The competition between rival books and documentaries to get in first has had the curious effect of publicising the anniversary four months early. Retired Det Chief Supt Roy Ranson led the murder investigation. His book, Looking For Lucan, was scheduled for publication in October. Last week it was brought forward to tie in with a Channel 4 True Stories documentary on the subject, to be transmitted next month before Lucan offerings from Granada and the BBC.
Ranson's book was also the subject of a battle for serialisation rights. When the Daily Mail published extracts last week, the Daily Express ran a 'spoiler' - its own serialisation based on Lucan Lives, a forthcoming book by Ranson's deputy, former Det Chief Insp David Gerring.
Ranson may be an expert on the case, but some observers have doubts about his consistency. In June 1975 he told Sally Moore, who wrote Lucan Not Guilty: 'I am convinced the sea somehow holds the secret to his death.' In his book, however, he maintains that Lucan is alive and describes visits to Mozambique, Botswana and South Africa to find him. Last week in an interview with the Independent on Sunday he was suggesting there was a 50-50 chance either way. But he maintained: 'I haven't changed my view. Being unable to find a body, it doesn't confirm the fact or possibility Lucan is dead.'
He explained that at the time of the murder he pretended to think that Lucan was dead to maximise publicity - a claim his former deputy flatly denies.
While Ranson and Gerring at least agree that Lucan is guilty, Granada has a more esoteric approach, recruiting a jury to decide. A mock trial of Lucan, starring Julian Wadham of Middlemarch fame, goes out in the autumn. Ian McBride, executive producer of The Trial of Lord Lucan, argues: 'Nobody ever really examines the evidence and seeks to find out whether he was guilty.'
Patrick Marnham, author of the 1987 book Trail Of Havoc: In The Steps Of Lord Lucan, points out: 'I think Lucan is dead, but if he were alive he would certainly have every grounds to object to his treatment from the 1975 inquest and now this mock trial. It's completely prejudicial.'
There are two other major players in this drama about a failed marriage and a tragic death: Lady Lucan, who is to contribute to a book by postgraduate student James Ruddick, and Mrs Rivett's family.
Lady Lucan maintained at the inquest that, although she did not see who murdered Mrs Rivett, she was shortly afterwards attacked by her husband, who forced his fingers down her throat, attempted to gouge out her eye and hit her about the head. The coroner, Dr Gavin Thurston, asked: 'You have no doubt it was he?' She replied: 'No doubt at all.'
But latterly she seems to have developed some, and in 1981 she implied in print that she could have been mistaken. This may be the 'new information' that Headline Publishing says she gives Mr Ruddick in his Lord Lucan: What Really Happened, due in October.
Lady Lucan's contribution will be interesting because she is the only person who could possibly answer many of the questions which hover so persistently over the affair. She has no contact with her children and attempted suicide 12 years ago. In 1983 she was committed temporarily to a Surrey mental hospital after being found wandering in a confused state near her home.
The final piece in the jigsaw are Mrs Rivett's relations and they, like Lord Lucan's children, have never talked to the press - undoubtedly forgoing huge sums of money in the process. When asked at the time of publication whether his book would upset Lady Camilla, Lord George and Lady Frances, all now in their twenties - Marnham retorted: 'What about Sandra Rivett's son? Mightn't he want to know who murdered his mother?'
But the truth is more complicated. Mrs Rivett at the time of her murder was separated from her security officer husband, Roger, who has never given an interview although researchers have been known to ring every Rivett in the phone book to try to track him down. Her father, Albert Hensby, refuses to speak, reportedly because his 21-year-old grandson does not know about the murder. But Mr Hensby gave a different reason to Channel 4 when they recently asked him to contribute to True Stories. 'I don't want to talk to anybody. My wife died of a broken heart,' he told the director.
The family's truest sentiments may be found in a comment made by Mrs Rivett's aunt during the inquest. 'I am appalled,' Vera Ward is quoted as saying. 'Sandra is the innocent victim here. She is supposed to be the central figure but her name has hardly come up.
'It is like the shopgirl and the prince. They make out Sandra to sound like rubbish but in fact she came from a good family background and was very refined. Sandra is in the middle of what seems to be a battle between the two sides of the Lucan family. Meanwhile it is Sandra who died and she is gone and forgotten.'
Sally Moore, who has passionately argued that Lucan did not kill Mrs Rivett, thinks the problem goes deeper. 'One of the injustices of the past 20 years is that the full truth about this case has not been able to emerge because of the stringent British libel laws. Masses of material had to be removed from my book for that reason. I would like to think that the whole truth will emerge this year - but I doubt it.'