i would question whether your gasman knows what he is doing has the fan and the air pressure switch are both very easy to test to see if they are perfoming correctly.
changing the circuit board seems to be a cure-all remedy that engineers use when they don't know whats wrong with the boiler. the fact is that the boards do not fail as often as you would think, there is no test you can do on a board, you test all the other components first and if they're all ok it only leaves the board.
i would be very very annoyed if i had spent 400 on a boiler that still doesn't work, in fact my customers would not stand for that at all.
its difficult to say what the problem could be without examining the boiler in person, the only things that make a clicking sound are relays, the solenoid on the gas valve and the ignition process, the air pressure switch does click but only very faintly so you wouldn't hear it.
if your in the south birmingham or surrounding areas give me a shout.
p.s. for the future, its worth knowing that potterton who are part of the baxi group offer an inclusive repair service with all labour and parts for 240 quid.