As for fitting the plugs, the engine wasn't totally freezing, but it had been sat off for a while. Torqued to 23N/M.
Doubt that would give this sort of "fault" though.
I've been Googling and it looks like I can use my Cobb AP to read the ECU for fault codes, so I might try that later. Will it show hidden codes, as nothing has come up on the dash...