Ravenheat LS 80 cuts out

[ratters]

When my boiler is switched on it fires up (not with a whooshing sound but with a sort of knock) but after about a minute or maybe two the flame failure switch comes on and the boiler goes off. The electrics are fine and the gas valve is working properly.
What could be causing this?

 

[suluki]

Turn your control to hot water only and fire boiler up if it works ok then turn controls to hot water and heat if boiler trips out again then i suspect your pump is knacked

 

[ratters]

Thanks but no it's not the pump. Any more ideas?
Getting desperate!