In this country, 70 per cent of a private detective's caseload is work that comes from the legal profession. These investigators tend to do the solicitors' dirty work - process serving, debt collecting. Only rarely do they crack the case.
The criminal law specialist Stephen Gilchrist has employed private investigators for a variety of reasons. But he doubts whether public money would have been made available to fund McKenna's work if Simpson had been represented by a public defender, or even if he had been tried in this country and had had to rely on legal aid.
In those circumstances, asks Mr Gilchrist, "Would thousands of dollars have been made available to employ a team of Humphrey Bogart lookalikes to pad around the streets of Los Angeles with the objective of dishing the dirt on the principal police officer in the case?"
Would, indeed, the Legal Aid Board have sanctioned a similar request to fund a private investigator to walk the streets of Gloucester in search of character assassination evidence against police officers involved in the Fred West case?
Mr Gilchrist believes all solicitors specialising in criminal work will come across a case that requires the services of a private investigator, but not all will be able to instruct one. "The principal reason for this," he says, "is that the resources are simply not available or could not be justified to public funds."
Solicitors cannot afford to run up costs in a case, vainly hoping that the Legal Aid Board will authorise the hiring of a private eye. Mr Gilchrist has encountered exactly this difficulty and found himself forking out from his own pocket.
"If you don't get prior authority, when you put your bill in, it's possible for the taxing authorities to disallow that expense," he says. Instead, he adds: "They should be more flexible in welcoming more investigations."
One hurdle confronting the LAB and the solicitor is being sure of getting value for money. Despite several attempts through Private Members' Bills at controlling their profession, private eyes remain unlicensed. Anyone can walk straight out of prison and become one. "We can't," points out Mr Gilchrist, "go to the police so we have to go to the private sector, which is unregulated and in some cases unprofessional."
Mr Gilchrist's experience is that the standard of work provided by private investigators varies greatly, not only between individuals, but between different work undertaken by the same person: "There is no consistency; some are very good, some are hopeless. The accreditation is worthless. There is no way they can be disciplined and there is no way of sorting the wheat from the chaff." He continues: "Larger outfits will take on anything and are better at doing some work than others. They will all imply that they have all these contacts. But at the end of the day they can come up with nothing."
But in an adversarial system you are entitled to challenge the case with whatever means are legally available. And a solicitor also has a duty to act in the best interests of the client.
Peter Helms has been a private detective since 1953 and is spokesman for the Association of British Investigators. He admits that the complete lack of control over his profession presents difficulties for the solicitor.
He advises: "It's up to the solicitor to go on reference and recommendation, but there are a number of solicitors who are so naive that if they get a well-worded letter from a private investigator they will instruct him without checking on his background." He says they then run the risk of being overcharged, lied to and even unwittingly involved in sanctioning illegal activity.
Mr Helms knows of cases where private eyes have committed perjury and have been bribed by defendants to withhold the service of documents. "The solicitor has to be wary because he is entrusting his client's business to the hands of a private investigator," he says. "My advice to a solicitor would be to instruct somebody who is a member of the Association of British Investigators."
The Law Society does not offer any specific guidance. For example, there is no register such as the one for expert witnesses. Alison Crawley, head of policy in the society's professional ethics department, says the only relevant practice rules are that a solicitor cannot do anything to break the law or bring the profession into disrepute. But, she says, "The best interests of the client require the solicitor to find the best private investigator for the job."
Solicitors cannot sidestep the issue. If the private detective says something to lead the solicitor to believe, or the solicitor simply believes that the investigator might do something to break the law, then the solicitor must act, says Ms Crawley. This is one area in which the society's professional ethics department receives inquiries from its members.
Solicitors who do not take pre-emptive action can end up with egg on their face when the case comes to trial. Ms Crawley recalls a personal injury claim where a private eye had deliberately let down the plaintiff's tyre; when the plaintiff started inflating it, the investigator took pictures to show the plaintiff was not as badly injured as he had made out.
But, Ms Crawley says, "Most private detectives are getting evidence which will later be used in court and most solicitors know that and make sure it has been properly obtained."
From now on, solicitors who do not take proper care when instructing investigators could find themselves in much more serious trouble. Mr Gilchrist warns: "Solicitors, along with anyone else who instructs investigators to obtain legally protected data, can be prosecuted under the 1994 Criminal Justice Public Order Act."