Well, I've checked the lambda probe, the heater seems OK and there is 12v getting to it. The engine does seem to rev a little easier with the probe disconnected, but the misfire is still there - in fact with my senses misfire tuned, I can also detect another at approx 300 rpm and a little hesitation when applying the throttle under load.
I've dismantled and cleaned all the ignition assembly, but can't see anything amiss. I guess the next stage would be to start swapping out the HT components.
I don't suppose it could be a dirty injector?
Anybody got any further suggestions?