I've just fitted the new double pumper carb that Bill at SA sent me and I'm very pleased to report that it most certainly seems to have fixed whatever the problem was.
I took it out for a short run and it's a totally different animal and also sounds different, much cleaner and more even.
I'm going to Le Mans in July, so I now need to get some test miles on it before the trip.
It's going to be a great weekend.
Thanks for all you help and advice.
Paul