potterton 16/22 not staying on?

i have recently drained and flushed the system and refilled it with a suitable inhibitor
but now the boiler is playing up

from cold the boiler will heat up in the normal way untill it reaches a point where the main burners will switch off,the bolier case at this point is very hot,however,the case fan and the pilot light remain lit,the boiler will not refire the main gas burners and as a consequence the radiators get cooler and eventually go cold and the hot water will only ever get to a tepid temperature