Sunday update (or at least, this weeks update)
Having got worried about the fuel pump (thanks Pete!), before attending the West Mids Meet this week, I went out and started the car to see if there was anything to see/hear and also to pop it into service mode.
Fuel pump primed just fine - about 2 seconds from ignition on to it stopping the full 'whizz'. Started fine (the revs still wobble up and down slightly). Popped it into service mode and the revs dropped right down, I'd say to about 500rpm. Turned the idle air screw round (and round) and eventually got the revs to about the right place - the fluctuation in revs at idle seemed worse. Out of service mode and the revs became more stable (but not spot on); I blame the HES, which I must get round to fixing. Anyway, seemed better but driving would tell. However, whilst standing there, looking at the engine ticking over and praying for inspiration, I noticed a water bubble forming and popping between the air intake hose and the sightglass on the header tank (YHO has a Coolex ali header). Closer investigation showed that the sight glass has failed and coolant is weeping out (and had pooled nicely on the tranny) and (I am guessing) some may have dripped onto the coils - not an ideal situation. Dear Coolex, please send me a new sight glass (which has subsequently arrived).
Took the car to the West Mids Meet and it was an 'interesting' experience; when cold, the revs were way too high and rather than engine braking (which I do quite a bit when bumbling along), it accelerated! Once warmed up though, much better. Made it there and back to the meeting, including giving it some beans on the way back (beans from my perspective, FredJohn was probably twiddling his thumbs behind me...

) and there were no hitches. I am still thinking it's a combination of HES, base timing, CO pot and idle screw that are in need of balancing.
Finally, it rained here today, so some inside work:
There are 10 kinds of people in the world; those that understand binary.....