Pilot light won't light on myson boiler?

The problem initially was that the pilot light would light but wouldn't stay lit when I let go of the button. the Plumber came round, flushed the system and replaced the overheat control dveice. This seemed to resolve the problem. However a few days later, the pilot light kept on going out. I had to keep relighting the boiler, however I noticed that after half hour the hot water header tank in the loft would overflow so I ended up turning the boiler off. This happened every time the boiler was on. The pilot light would then not light until all the air had been bled from the radiators. The pilot light will now no longer light at all. Does anyone have any ideas?