The above boilers which have been recommend are very expensive for spares & some spares would be special orders as many merchants wont have them in stock. I would advise if you are looking for a decent cheap boiler a baxi 105he which is also the same boiler as a potterton performer or Main...
1.Glowworm have now upgraded filling loop due to let-by problems.2.water pressure problems,When heating has reached temp check pressure gauge has not gone upto 3 bar.otherwise you have a leak on the system somewhere
flow switch is normally the problem.the way to test this is to pull flowswitch of divertor & push in white button on flowswitch and see if it then fires up.if not try both hw/ch thermistors.alphas are good boilers but can be exspensive for parts.invest in a bg 3* contract