Glowworm 40ff boiler not lighting/firing?

Hi, I have a glowworm 40ff boiler that is not firing, - the pilot light is there and its sparking but just not firing. it had been working intermittently but now just wont fire at all.

I have had an engineer look at it who said it was something to do with the voltage not being enough & he cleaned the contacts for something (I cant remeber what) and that seemed to work for a couple of months but not its gone again. I have just purchased a new pcb for the boiler but its made no difference!! Any ideas?

Thanks