The fan operates a pressure switch and the switch is the signal to begin lighting up. The PCB controls it all.
Maybe the fan is not running as it should
Maybe the pressure switch is faulty
Maybe the Flue or the case seal is faulty
Maybe the PCB is not working properly
Maybe the Gas Valve is defective
A heating engineer can make an accurate diagnosis - eventually...
Good luck...
Peccavi
June 2009