So, hilariously it appears that every lumpy/uneven running issue, every moment where the car seemed a bit hard to start, every time it felt like something was a bit untoward in the engine compartment...was purely down to some of the bits marked 'Lucas'. Because replacing the Lucas plug and coil leads with Bougicord ones has solved the lot.
The Prince Of Darkness™ strikes again. WHO COULD POSSIBLY HAVE FORESEEN THIS???
And I mean it - it's better enough now that we might not even need to replace the distributor and coil with EI-spec kit. Gonna give it a couple of weeks and see how we go, but take tonight - it's poured with rain nearly all day and it's not been what you'd call warm, but the car fired off clean with the choke cable pulled halfway and ran fine off-choke within a minute. Of course, I do now have to un-arse the carburettor settings and re-do them from scratch because I've been tuning around the ignition issue (I'll be charitable and say that the engine is running a
tad rich right now, in fact it's absolutely pig fat
).