I'm going to try to get it on a rolling road this week if at all possible.
Thanks for all the suggestions, I really do appreciate it.
I re-routed the distributor leads around the opposite side of the engine bay and away from the coil leads and there has been an improvement. However, I also changed the vacuum diaphram spring for a weaker one, so I don't know which change made the improvement
I suspect the #3 plug was red herring and may have been down to a misfire caused by the parallel running of the coil and dizzy leads. I've looked at it again and it now looks normal.
I'm still inclined to think it is a fuel issue as it also won't pick up cleanly.
Paul