Update, I put those two extra bolts in and the misfire's gone. I started it 3 times (and drove it about a bit) by which time normally the EML goes off but this time it didn't. Maybe I have to leave the engine off for longer in between.
Anyway, if the misfire's gone at present, but if it keeps coming back that suggests very strongly a dicky ECU, to me.
If it goes and stays away I'm still going to go poking around and clean the throttle body and the injector valves if I can get to them. I'll breathe a sigh of relief and spend the last of my money on the brakes and steering instead of the electrics.
Thanks, Robin.