could also be blocked flue, venturi or air pressure tubes, faulty air pressure switch, faulty pcb, or faulty harness. get a qualified engineer in, it'll be cheaper in the long run.
Ian
April 2010
turn on/off knob to R hold for 5 sec and see what happens
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.