Thanks very much. I'd missed that, probably because I was expecting it to be a microcontroller (although I'd got as far as thinking it odd that there was no sign of a crystal or resonator).
I might end up sticking an analyser on it out of interest: it's collecting around 16 switch contacts and putting them onto a serial bus (believed to be 24-bit packets with clock, data and direction signals). It's controlled by a P38 Range Rover's BECM module, probably not directly visible to OBD.
MarkMLl