Ok, so now done about 200miles. All is going well except that I'm getting exceptionally bored staying below 4000rpm.
Did the timing and tuned and balanced the carbs yesterday and have found that it starts a little bit easier - timing was advanced quite a lot which could explain some of the difficulty in starting.
But it's still not starting very well so I've bought an overhaul kit for the Bosch starter from EuroMoto Electrics. Will put that in next weekend and see how that goes. If that doesn't fix it I'll probably buy one of the repro Valeo starters that EME sells as well as some better quality leads. Hopefully that will sort it out.