28i junior CH problem?

I have a fault with the central heating of my worcester 28i junior. The boiler fires up and the radiators heat up, but after 5 min or so the red light flashes slowly and the boiler stops.
The air pressure switch has been replaced and is different to the original, could this be related? I have checked the radiators and there doesn't appear to be any air in the system. The water pressure is about 1.5 and doesnt increase greatly when running.
Any help would be appreciated

stan
November 2008