problem with glowworm 50bf (balanced flue)?

pilot lights fine. when calling for both heating and hot water pump runs but burner will not ignite or should i say very rare as sometimes it will, you here gas valve click and it fires but only on the odd occasion. this is a balanced flue not fanned with an old multifunctional valve with a nob to change to pilot on or off, electric supply seems fine to the boiler i can only think of gas valve, With the boiler being resonably simple but i cant be certain any advice would be a apreciated.