What could cause the burners not stay lit on Worcester RFS 280 System?

My central heating system (Worcester RFS 280) the main burners won't stay alight after the pilot light has ignited the main burners for more than 3 secs. The hot water system works ok. What could the problem be caused by? My thoughts that it is the main control board, I don't want to replace it if its not the cause, suggestions welcome