baxi bahama 100 will not ignite?

I have had the flow valve replaced in the boiler, this being the one that fills the boiler itself. for some reason it was restricting flow that resulted in low water pressure. having resolved that problem - I just can't seem to get the boiler to ignite. the pressue is there, and I can hear the boiler trying to fire up when I turn the hot water tap on, but no ignition.