This has all been really helpful. Thinking about it a little, the performance was a little haphazard in the last few days of running in the autumn...which I now realise, could be the early signs of spark plug degradation, though I had no idea at the time..... Yesterday, we went on a 70 mile run, around Winchester, Southampton, Hamble etc and the engine was a lot smoother that I can recall for ages. No spits or overrun and a nice smooth eager acceleration profile. I have ordered a flow meter and will attempt to adjust the air flow to balance up the carbs....there's definitely a little work to do as she's a tad lumpy at tickover still.