It'll be the pump (its always the pump) - unless its the (sorry spelling) pieztrometic sensor. Either way its an easy fix.
Unless you have a new boiler - in which case, as any surveyor will tell you, they're all crap.
Inherited the boiler with the house 12 months ago. Its a Worcester combi and is likely to be less than 5 years old.
I have managed to get the burners to light but they go out again after about 15-20 secs.