When Mohamed al-Fayed takes the stand tomorrow at the inquest into the death of his son, Dodi Fayed, and Diana, Princess of Wales, he will be expected to explain to the coroner his "conspiracy" theories about how the couple were "assassinated". However, he will have to pick his way through his fallen hired hands, who have failed to storm, or even approach, the ramparts of inquest credibility.

If Fayed thought for a Harrods nanosecond that his previously reported views would remain unchallenged by what he perceives as the establishment, last week would have been deeply disappointing. Fayed watched glumly as Lords Jay, Fellowes and Stevens produced solemn evidence to demonstrate that it had been impossible for any of them to have been part of an assassination or cover-up. Sir Richard Dearlove, the former MI6 chief, will take the stand after Fayed, in what may prove to be the most dramatic week of the inquest so far.

Last week opened with Fayed's latest PR spinner, Katherine Witty, being condemned by the coroner for her "inappropriate behaviour" in court.

The jury, seated directly opposite Fayed and his entourage, had apparently been disturbed by a smirking Witty during Michael Mansfield QC's cross-examination of Detective Sergeant Philip Easton. Easton had travelled to France as part of Lord Stevens's Paget inquiry into the deaths.

It has always been Fayed's belief that the late James Andanson, a photographer, had been driving his Fiat in the Alma tunnel at the behest of MI6 and the Duke of Edinburgh to assassinate his son and Diana.

In fact, Sgt Easton reported, the French had established that Andanson had been in his marital bed 99 miles outside Paris when the crash happened. Pictures of the charred Mercedes in which Andanson later committed suicide in 2000 were shown to the jury. Witty's smirking during the proceedings was reported by the jury to the coroner.

His rebuke to Witty was mild in comparison with the punishment dispensed to Fayed's lawyer, Stuart Benson, and his personal security chief, John Macnamara. The Metropolitan Police QC, Richard Horwell was primarily responsible for the legal savaging, aided by the coroner's legal team, led by Ian Burnett QC.

The former Metropolitan Police commissioner, Lord Stevens, looked witheringly at the Fayed camp and spelled it out, "We want an apology". Fayed had accused Stevens and his Operation Paget team of Scotland Yard detectives, who spent three years failing to find any evidence of a conspiracy, of being part of "the establishment cover-up". There is no more serious charge against a group of career police than accusing them of covering up murder.

Lord Stevens might never get his apology, but his Paget detectives, some of whom were in court with him on St Valentine's Day, took some satisfaction in watching Horwell take McNamara apart. Lord Justice Scott Baker forced Macnamara to admit that he had lied in public about the alcohol consumed in the Ritz hotel by Henri Paul. "If you tell lies on some occasions, how can they [the jury] tell if you are telling the truth on others?" Macnamara singularly failed to produce any evidence to support any of the conspiracy claims. Horwell forensically dissected the remainder of Macnamara's evidence.

The coroner also wondered if Macnamara had apologised to former bodyguard Rees-Jones for making the "very serious allegation in your statement", 'in my opinion, Rees-Jones has willingly and in return for payment been used as a mouthpiece by or on behalf of the security services to try to discredit the mounting evidence that the crash was not a simple accident'."

On oath Macnamara withdrew this claim, but claimed that he had not been able to apologise to Rees-Jones, as he had not seen him since he made it. I was surprised by this, as I had witnessed the two men acknowledge each other in the High Court on 29 January, while Rees-Jones prepared to give his evidence.

Stuart Benson, Fayed's personal solicitor for nearly 20 years, became another victim as the inquest considered his "evidence" that Princess Diana was engaged to Dodi – a key element in any conspiracy argument, as it would provide motive for murder.

Benson claimed that the couple could have been engaged as result of receiving a phone call from Dodi during their last holiday.

A sceptical Horwell first teased Benson: "In matters of love and matrimony, Benson, the lawyer is not normally the first person that the groom contacts, is he?" Then the coroner asked Benson if he thought he might have been "gilding the lily", before Horwell devastatingly reminded him of an earlier action. The judge in that case, Mr Justice Mann, said he was reluctant to accept "any evidence" from Benson.

Mann also said Benson was "... reluctant to give evidence that he calculated might be against the Fayed case, yet capable of exaggeration if that case required it".

Benson could not remember when he had conveyed the exciting "engagement" news to Fayed, or why he had never sought to correct his spokesman, Michael Cole, when he told the world, on the eve of Diana's funeral: "What that ring meant, we shall probably never know... if the planet lasts for another thousand years."

Michael Cole himself, Fayed's veteran spinmeister-in-chief, had been an earlier victim. The coroner's QC reminded him that while he now claims to have known about Princess Diana's "pregnancy", he had, very publicly, condemned the press for "giving currency to nasty rumours, which have no factual basis".

Horwell produced reams of 1997/78 correspondence from Cole on Harrods writing paper to the Press Complaints Commission in which he argued that she was not pregnant. Oops.

Martyn Gregory is the author of 'Diana The Last Days' (£6.99 Virgin Books) and Sky News's expert commentator on the inquests