Thanks Mark,
Found some info last night (just sometimes it would be nice to switch off and watch a bit of telly and not bring the work home).
Ok so now I've got the accelerometer on the back of the block detecting combustion noise, a development of a knock sensor I should think.
Also I know now this system is likely to have a fuel pressure control solenoid on the end of the rail. That'll be fun to get at on this engine, can't even see the rail let alone the ends (even with a mirror).