I agree with nigeldtr a new SRM oil pump solves the problem, and as these bikes require great care and attention a new oil pump not only solves the problem but as said, gives that all important oil delivery to the engine. Fitting anti sump valves is just a problem waiting to happen and as bill says you cannot rely on you memory. Ok so a new pump is £225 but that is a very small percentage of the motorcycles value and will give year of trouble free service.
All the best Goldy