Potterton Prima 40C - Pilot light on, no fan/main burner?

Help! No heating!

It was working perfectly until yesterday. The pump's working ok, and comes on when the CH is switched on, but the boiler does nothing. The pilot light is on, but the main burner does nothing and the fan doesn't switch on. My first thought would be PCB, but I'm no expert and am reluctant to spend that sort of money without some more expert advice. Any thoughts?