Combustion analysis carried out by a QinetiQ scientist, on behalf of the BOI team, has confirmed that high altitude fuel ignition by electricity is impossible without High Energy sparks. This leaves only pure heat as the cause of the ignition. The only heat source near to fuel pipes and tanks in a Nimrod are the engines (with fire extinguishers) and hot air pipes. All hot air pipes that are not in the immediate vicinity (and therefore protected by the fire extinguishers), have not been in airborne use since 2 Sep 06. This includes the SCP, the pipe that supplies it and the bomb bay heating system. It is considered that the bomb bay heating system pipes do not carry air hot enough to ignite any fuel playing onto them, so that system might be brought back into service.
There was a comprehensive post-accident fuel system fire hazard analysis carried out by QQ in 2007. It confirms that, with the limitations put in place within days of the accident, there is sufficient mitigation against a fire caused by a fuel system leak. To quote someone, " The Nimrod is as safe as it needs to be". He was correct.
It is the simple fact that the RAF was able to mitigate against the cause of a fuel fire within days of the accident by procedure alone (and was therefore capable of doing so before the accident, but didn't, for reasons yet to be established) that the S of S had no choice but to say sorry and offer compensation. We knew, with our radical and far-reaching limitations, that we had eliminated all possible causes, although we didn't know the probable point of ignition until the BOI reported its findings.
Ed