THE ''headless man'' has finally developed a face. Forty years after the scandal which rocked the Swinging Sixties, film star Douglas Fairbanks Jr will tonight be unmasked as the notorious figure in the sexually explicit pictures at the centre of the Duke and Duchess of Argyll's divorce trial.

According to evidence in the 1963 Denning Report which was not published at the time of one of the most sensational divorce cases of the century, Fairbanks Jr was the headless man despite years of denial by the swashbuckling star and his lover, Margaret, Duchess of Argyll.

The results of the inquiry are revealed tonight in a Channel 4 documentary which also uncovers evidence of a second headless man.

Margaret Argyll, the only child of self-made multimillionaire George Whigham, was the society beauty of her generation. Her first husband was a rich American called Charles Sweeny.

As Mrs Sweeny, the glittering socialite, she inspired Cole Porter to write: ''You're the top, you're Mussolini, you're Mrs Sweeny, you're Camembert''.

She had two children by Sweeny and after their marriage foundered in 1951 she married Ian, Duke of Argyll, chief of clan Campbell and hereditary Master of the Royal Household in Scotland.

The documentary says Fairbanks was among a list of 88 men the duke had thought to be among his wife's lovers. They included Government figures, members of the Royal Family, and film stars.

The sexually explicit pictures, which show the duchess performing a sex act with the ''headless man'' in the gilt and silver bathroom of her Mayfair home, were a key piece of evidence in the divorce.

The judge, Mr Justice Wheatley, said the duchess indulged in ''disgusting sexual activities to gratify a basic sexual appetite''.

Since then, the mystery has remained, although there has been fevered speculation about the man's identity.

Fairbanks, who died earlier this year at the age of 90, had been named as a possible suspect but continually denied any involvement. The duchess remained silent on her lovers but the fallout from the scandal spilled over into the political world.

The divorce case was heard in the same month that John Profumo lied to the Commons about his relationship with Christine Keeler. As the shock waves crashed over society, Margaret was branded an upmarket Keeler.

Early in June 1963, Profumo admitted his deception and resigned. Before the month was out, however, the Macmillan Government faced a new threat.

After a stormy Cabinet meeting on June 20, Lord Denning - who had been commissioned to investigate the security implications of the Profumo affair - was asked to look at claims that Duncan Sandys, the Cabinet Minister and Winston Churchill's son-in-law, was Margaret Argyll's headless man.

Sandys, who was given a peerage to become Lord Duncan-Sandys in 1974 and died in 1987, allegedly said of the rumour: ''There's safety in numbers.''

There were hand-written captions on some of the erotic photographs the duchess had, so Lord Denning tried to match the handwriting.

He invited the five key suspects - including Sandys and Fairbanks - into the Treasury, where he asked for their assistance in a ''very delicate matter''. As they arrived, each had to sign the visitors' register. Denning then employed a top graphologist to analyse the handwriting.

The results were conclusive, as a young Treasury official who assisted Lord Denning at the time, the broadcaster Peter Jay, recalled: ''The headless man identified by the handwriting expert and therefore identified by Lord Denning, though he didn't write this down in his report, was in fact the actor Douglas Fairbanks Jr.''

Sandys appeared to be in the clear but there was another photograph in addition to the set of four of Fairbanks, in which the duchess - apparently naked apart from her trademark pearl necklace and a prominent ring - was shown ''performing a sex act'' on a second headless man.

This photograph is revealed for the first time in the programme. With no caption on the bottom, it was clearly not from the same set as the others and the physical characteristics of the man with the duchess appeared different from those of Fairbanks.

The duchess seemed set to take the secret of the identity of this second man to the grave but, just before her death, gave away a vital clue to her close friend Paul Vaughan.

He said: ''She did say to me quite clearly that, 'Of course sweetie, the only Polaroid camera in the country at this time had been lent to the Ministry of Defence'. If that wasn't running a flag up the flag pole I don't know what was. She wanted someone to know.''

Since at the time the photographs were taken - the Polaroid film stock was dated to 1956 - Sandys was serving as Secretary of State for Defence, the programme team believes there can be little doubt Margaret was hinting to Vaughan that Sandys was her second headless lover.