This report quite possibly contains the smoking gun on the deal. It is (as far as we know) the only NAO report to have been completed and yet not published.

It dates back to Sir John Bourn's early years as Comptroller and Auditor General - indeed it is possible that it was commissioned by his predecessor, Sir Gordon Downey. According to the Camapaign against the Arms Trade

The NAO investigation took three years, and in March 1992 the House of Commons Public Accounts Committee (PAC) agreed not to publish its findings. The Chairman, Robert Sheldon MP, refusing to disclose the report even to the Committee members, simply assured them that there was "no evidence of fraud or corruption". It seems clear that the inquiry had proceeded within narrow limits: Sheldon acquitted the MoD alone of having made improper payments, finding that "the deal complied with Treasury approval and the rules of government accounting" and that "there was no misuse of public money" (Independent, 12.3.92, 24.6.97). However, the NAO only investigated the MoD; as Sheldon states, "We were not able to follow money outside the department once it is paid to the contractors, so we do not know what was done with it" (Independent, 24.6.97). Sheldon made it quite clear that the reason the report was not published was the "highly sensitive situation regarding jobs in the defence industry". (Independent, 12.3.92) Later he was even more specific: "The Saudis would have been upset". (Independent, 23.6.97)
The PAC decided not to publish the NAO report, despite the fact that most of its members were not even allowed to read it. Sheldon invited a Conservative member, Sir Michael Shaw, to read the report and to join him in interviewing Sir Michael Quinlan, Permanent Secretary at the MoD, and Sir John Bourn, head of the NAO.
The non-publication of the NAO report meant that it was impossible to dismiss charges that commissions had been paid. Members of the PAC were not happy. As Alan Williams MP said, "quite a few of us [on the Committee] had misgivings about the suppression". (Observer, 10.5.92) Dr Kim Howells MP said that the situation was "most unsatisfactory. If we can't see the report, and it goes right to the heart of the problem, what does the PAC exist for?". (Observer, 10.5.92) A former member of the PAC, Jeff Rooker MP, who had pressed for the investigation in 1989, said that he was "astonished" by the decision; "the committee is supposed to be independent of political considerations such as jobs". His colleague Dale Campbell-Savours MP, who had served on the committee for eleven years, was "convinced that payments had been made". (Independent, 12.3.92)

The police team investigating the Al-Yamamah arms deal asked for the report earlier this year, but were refused, amid an unusual debate. One Public Accounts Committee Member, Harry Cohen MP, accused Sir John Bourn of a "serious conflict of interest. Sir John did a lot of work at the MoD on Al Yamamah and here we now have the NAO covering up this report."*

The Liberal Democrats are calling for publication of the report now. The Guardian report Ming Campbell as saying "Parliament is entitled to see any report commissioned in its name. There is no reason why this report should be treated any differently."

It will be interesting to see whether the Conservatives support moves to publish this report - which might just throw some light on the constant rumours that Mark Thatcher was involved in the deal

*Prior to his appointment as Comptroller and Auditor General (agreed between Robert Shedlon and Margaret Thatcher, Sir John was Under Secretary for Defence Procurement.