Overkill. It may be in poor taste but it is surely the best pun in the dictionary to sum up the mawkish and farcical charade unfolding in court 73 of the Royal Courts of Justice in The Strand. The most abiding mystery about the death of Diana Princess of Wales is how the English legal establishment was conned into holding a third lengthy inquiry into it at considerable public expense to satisfy the paranoid obsession of Dodi al Fayed's grief-stricken father.

Some sections of the media are delighted, of course. A small Swedish forest was sacrificed to enable the Daily Mail and Daily Express to bring us Tuesday's vague and contradictory evidence from flaky flunky Paul Burrell. More than a decade after the fatal accident, they managed three pages apiece yesterday.

I have the greatest respect for the institution of the coroner's inquest. It has its roots in Anglo-Saxon England and its primary purpose is to answer four simple questions about a death: who, when, where and how? Three decades ago, as a trainee journalist in Cardiff, I remember attending the inquest into the death of a young Asian doctor who drove off the road and into a tree after working for 20 hours without a break. In my memory I can still hear the quiet weeping of his beautiful young widow, as a pathologist recounted details of his torn and broken body. It felt like a cruel intrusion into private grief but it performed a necessary function. It resulted in recommendations about the travel arrangements of overworked junior doctors. That inquest took a single afternoon.

Lord Justice Scott Baker is the fourth coroner to take on the Diana inquest and is reported to be attempting to contain it to six months. Somewhere along the way this event has come off the rails of proper judicial scrutiny and entered the world of fantasy.

Instead of "Who, when, where, how" - questions that could all have been answered satisfactorily within a week of her death - the coroner tells us (on his website) that he is tackling a range of issues, including "whether driver error . . . caused or contributed to the cause of the collision"; ditto the actions of the pursuing pack of paparazzi; "whether the Princess of Wales and Dodi al Fayed were about to announce their engagement" and "whether the British or any other security services had any involvement in the collision".

Even this could not have prepared us for the tawdry display of necro-pornography that has emerged from court 73: details of Diana's periods and contraceptive arrangements; a photograph of her with a "prominent stomach" in a leopard-print swimsuit that turned out to have been taken pre-Dodi (an irrelevant intrusion into an irrelevant question); a minute dissection of CCTV footage of the "happy couple" entering a lift, minutes before they died. And in the midst of it all, teasing the whole thing out and acting as mouthpiece for Harrods' owner Mohamed al Fayed's absurd conspiracy theories, is Michael Mansfield QC, once a hero of the left - the man who spoke up for the Birmingham Six and the parents of Stephen Lawrence. Rarely can his old adage about "defending the indefensible" have seemed so apt.

Presumably, the coroner's logic is that by shining a light into every corner of this sordid affair the crackpot conspiracies finally can be debunked and Diana laid to rest. If so, like a lot of the characters in this melodrama, he is deluding himself. The French authorities spent two years investigating Diana's death, interviewed 200 witnesses and prepared 6000 pages of evidence. They concluded it was a tragic accident. So did Lord Stevens after a three-year British investigation that produced an 832-page report and cost British taxpayers £3.7m.

Anyone who still thinks the couple were murdered by MI6 on the instructions of Prince Philip has been consuming too many historical novels or Hollywood blockbusters.

English law only requires an inquest into the death of someone who has died abroad if the body is returned to Britain for burial. It is usually a formality. Instead, this one has gone through four coroners and is likely to cost £10m. This includes a two-day visit to Paris by private jet for the jury and security escorts to and from the jurors' homes every day. Why?

It's easy to be smug, but it's hard to believe that this nonsense would have been allowed to unfold in Scotland. Fatal accident inquiries under a sheriff are the Scottish equivalent of the coroner's inquest. As well as covering deaths at work or in custody, they can be called by the Lord Advocate on the grounds that the death was sudden, suspicious, unexplained or gives rise to public concern. The biggest in recent years was for the Lockerbie disaster in which 270 people lost their lives. It took 61 days and cost £3m. In the Diana inquest that sum will barely cover security.

It appears that an extremely wealthy man has been allowed to hoodwink the authorities into giving him a platform for a theory that more befits Dan Brown than a court of law.

As a conspiracy theorist Mr Fayed is only outdone by the paranoid delusions of Diana herself. Her claims to her lawyer include, inter alia, that the Queen was about to abdicate and that both she and Camilla Parker Bowles would be somehow removed from the scene in favour of the royal nanny, Tiggy Legge-Bourke. No wonder this inquest has been described as "Big Brother in ermine". It is a mockery of the legal process.

Why did Diana die? Dark forces? No. Probably because she wasn't wearing her seatbelt. There's a lesson there but it doesn't take £13.7m of taxpayers' lolly to work out what it is. Harriet Harman, one of the ministers responsible for attempting to reform the inquest system, admitted in 2006 that it is "variable in its processes and its quality". That's an understatement. While the super-rich can hire top QCs and make wild accusations against those in no position to fight back, ordinary bereaved families have to fight for a paltry slice from the fast-disappearing legal aid cake.

Dorothy Wright from Largs was charged £577 by a coroner to see the harrowing account that will be presented at the inquest into the death of her son, Mark, in an explosion at a scrap metal firm in Chester in 2005. It is cases like these that ought to have formed the basis for reforming corporate manslaughter legislation. Instead, she is having to fight for justice with other bereaved families who believe their loved-ones perished as a result of workplace negligence.

A poignant post-script. If the Diana inquest overruns, one casualty will be the inquest into the death of Jean Charles de Menezes, mistakenly shot by police marksmen at Stockwell tube station, because Michael Mansfield is booked to represent his family, too. It says it all, doesn't it? Come what may, the Diana and Dodi show must go on and on and on.