I use the Calor people to service my Calor boiler.
I could have a service agreement, but the man said it would probably work out as cheap to pay as I went so that's what I do (I don't believe in buying insurance unless it's for something I wouldn't be able to afford if it happened - like a house fire, or a 3rd party accident claim)
I agree that it's hard to find a good plumber, and when you've found one they retire and hand over to a son who may not be as good.
All the good workmen, plumbers, builders, garages seem to have been snapped up by the insurance companies so how can we find a reliable 'little man'.