It must have been an eye-popping telegram. When Sir Sherard Cowper-Coles, the unflappable ambassador to Saudi Arabia, told his bosses in London that Riyadh was threatening to scrap all security ties, including intelligence-sharing on al-Qaeda, it appears he sent Downing Street into a tailspin.

For much of the autumn, the Saudis and BAE Systems had been putting pressure on the government to stop a long-running Serious Fraud Office investigation into the al-Yamamah arms deal, Britain’s biggest export agreement, negotiated by Margaret Thatcher in the 1980s.