I eventually found my original problem was that the timing had slipped on the distributor, probably on that run on new years day. Took a while to figure it out but trial and error got the bike working properly again. In future I will use the fully advanced position for timing as I think the static timing at tdc makes it too advanced - maybe the bob springs are weak when running on tickover.
Anyway - Simes might try your trick with the extra float valve washer and see how the mixture goes
Also I've been busy in other areas - that old sunbeam now runs and sounds fantastic ! I just de-glazed the bore, fitted new rings and made a copper head gasket.. certainly made me smile when it burst into life.