I think I have at last found the cause of the problem. The oil level was very high and the fuel tank almost empty. It was half full when I did the lay up. Could the fuel have leaked into the cylinders and past the rings and into the crankcase ? It is running perfectly now but I need to service or replace the fuel taps to be sure it doesn't happen again.Does anyone have or know where I can get rubber ( fuelproof ) washers ?