Failed 4th part of the fault-finding guide, which meant ECU.
Tried a fuse out reset to no joy, so took it apart and couldn't see a single dry joint (really, this was way better than even the JDM one had been, there was NOTHING visible wrong with any of them). Re-soldered various bits anyway, slaved it back in and to my absolute amazement, it worked fine.
Properly re-fitted it and was fine on the 400 miles drive yesterday.
Very odd, but very pleasing!
