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.   
  
thermostat i.e eletronics i.e expense for no good reason - sorry   
  
if it's firing, some annoying bit of electronics is turning it off - you have fuel, combustion etc 
  
And this normally means money. The engineer will just come in and change an entire electronic circuitboard. Unless it really is something else.