Potterton Promax 15HE not working
 

Dunno what's gone wrong, but I do know it's bloomin cold without it!!
I fitted the boiler about 3 years ago and it's all been working fine until about 3 weeks ago. THe only way to get it running again is to reset it. That was fine - happened once or twice a week at first. Friday when it stopped 4 times in 2 days.
Gave it a service as per the manual - everything looks fine, and I gave it the good news with the hoover. Condensate trap looked a little full of debris, but that got a clean out. But to no avail.
How can I go about troubleshooting this? The pump is definately running, and everything is calling for heat but the thing doesn't fire.

TIA,
David.