Detective firm invoiced Mirror 230 times in two years
Most of the Hackgate coverage has been about alleged illegal behaviour by News of the World journalists and the private detective it employed, Glenn Mulcaire.
However, there is a related issue which will be probed by Lord Justice Leveson, who has been asked by the prime minister to examine the culture, practices and ethics of the press.
According to sources close to Lord Justice Leveson, he will be looking at the extent to which newspapers used private detectives instead of journalists to ferret out information.
He is expected to take a view about whether this is an appropriate way to behave, and whether readers should be informed that stories have been obtained by hired detectives rather than through more conventional journalistic enquiries.
It is in that context that the BBC has been investigating the employment by the Daily Mirror in the late 1990s of a controversial firm of private detectives, Southern Investigations, whose boss Jonathan Rees was jailed in 2000 for conspiring to plant cocaine on an innocent person.
What the BBC has learned is that Southern Investigations was employed by the Daily Mirror and Sunday Mirror on 230 occasions between 7 October 1997 and 23 September 1999. The total sums billed to the Mirror by Southern Investigations were just under £67,000.
To be clear, there is no evidence to suggest that Southern's work for the Mirror was illegal - and Mirror group has not denied that it hired private detectives. Other media groups, including the BBC, have also employed private detectives.
But it is very unlikely that the Mirror's readers had any knowledge that Southern was such an important resource for the daily and Sunday tabloids. On average, Southern Investigations was doing two pieces of work per week for Daily and Sunday Mirror in the late 1990s.
This routine and systematic use of private detectives is of relevance to the Leveson enquiry.
Among the activities carried out by Southern for the Mirror included what an invoice of October 1998 described as "our motor cycle surveillance operative maintaining observations on your behalf" of the broadcaster Kirsty Young.
The bill for the surveillance of the former newsreader, now the presenter of Radio 4's Desert Island Discs, was £307. This kind of activity is legal.
Also among hundreds of other invoices were £264 in March 1999 for "confidential enquiries" into Mick Jagger, £25 for "assisting" with enquiries into George Michael in October 1997, and £668 in October 1998 for information on the business interests, bankers and "lender/borrower" details of the former rugby international Will Carling.
Again there is nothing to suggest these activities were unlawful. But Mr Carling, whose mortgage details were included in a Mirror article of 7 October 1998, told the BBC: "I'm pretty staggered they could publicise personal financial information."
Vast numbers of other invoices detail work by Southern to find out private information relating to many other prominent or newsworthy individuals.
Another of Southern Investigations' invoices, sent to the Daily Mirror and dated 26 August 1998 for the amount of £174, was for supplying a mobile phone number and the pin number for accessing voicemails. The owner of the phone is not known.
This information could have been used to hack into the voicemail of that mobile phone - although there is no evidence that it was used in this way and the Mirror and its editor of the time, Piers Morgan, have denied they engaged in phone hacking.
Phone hacking was not illegal until 2000.
What has already been reported, by the BBC and others, is that in this period Southern Investigations obtained mortgage details of the current governor of the Bank of England, Sir Mervyn King, and members of the Bank of England's monetary policy committee (MPC).
What is striking, however, is the detail obtained by Southern of the finances of Sir Mervyn and his colleagues. The BBC has seen notes made in 1998 by Southern Investigations of the mortgage balances, monthly mortgage payments and mortgage interest rates paid by Sir Mervyn, and by two of his MPC colleagues of the time, Deanne Julius and Willem Buiter.
Lawyers and bankers have told the BBC they cannot see how this confidential financial information on Sir Mervyn and his colleagues could have been obtained lawfully.
The information was used as material in a story that ran in the Mirror about how decisions on interest rates made by the monetary policy committee would affect the personal finances of committee members.
I spoke to Mr Buiter and Ms Julius, and they both told me they wondered at the time how the Mirror had obtained the information.
Also among the Southern Investigation documents are details from late 1998 of the mortgage balance, monthly payment and interest rate on Peter Mandelson's mortgage account at the Britannia Building Society, and also details of the balance of his current and savings accounts at the bankers Coutts, together with a list of the direct debits on his account.
Again, bankers and lawyers have told the BBC they can't see how the information about Lord Mandelson's savings and loans could have been acquired lawfully.
That said, many journalists would take the view that there was a powerful public interest in obtaining information about Mr Mandelson's personal finances, given that he had received a secret loan from a fellow member of the government, Geoffrey Robinson, which supposedly created a financial tie between the two that should have been disclosed
The details of Lord Mandelson's savings held at Coutts were published by the Mirror on 24 December 1998, in an article alleging that when obtaining a mortgage from the Britannia Building Society Mr Mandelson failed to divulge to the society that he had also borrowed £373,000 from Mr Robinson.
Mr Mandelson was cleared of any wrongdoing in relation to the Britannia mortgage.
In early 1999, Southern Investigations sent four separate invoices in relation to its investigations into Lord Mandelson totalling £1,116. The invoices say, among other things, that the work was for "confidential enquiries" into Lord Mandelson and into his parents.
Many would argue that there is a public interest for private detectives to be employed to uncover information relevant to a judgement about the probity of politicians or public figures, if it is impossible to be obtained in more conventional journalistic ways.
But there will be some who will question whether private detectives should be routinely hired by the media to find salacious gossip about celebrities or to uncover information about vulnerable individuals - as happened at the News of the World.
The founder of Southern Investigations, Jonathan Rees, was jailed for seven years in 2000 on charges of conspiracy to plant cocaine on an individual, after the period during which he worked for the Mirror.
He and his firm also worked for the News of the World. Between August 1998 and September 1999, Southern invoiced the now closed Sunday tabloid 66 times for a total of £13,000. All but one of the invoices for the News of the World were addressed to Alex Marunchak.
After Mr Rees was released from prison, he again provided paid services to News International, owner of the News of the World - as was recently revealed by Panorama.
Documents seen by the BBC show that between 21 October 2005 and 8 September 2006, News International paid Mr Rees a total of around £6,000 for work itemised on 16 separate News International self-billing tax invoices.
The Mirror Group said: "Many years ago some of our journalists used Southern Investigations. They were last used in 1999. Trinity Mirror's position is clear. Our journalists work within the criminal law and the PCC code of conduct."
Mr Rees's solicitor, Nigel Shepherd, of Cousins Tyrer, said: "Mr Rees has carried out lawful enquiries on behalf of most media organisations including… the BBC."
The BBC has conducted a search and said it could find no record of Mr Rees having helped with a broadcast or having been paid by the BBC.