Thank you for your advice. I had the boiler serviced in May but they couldn't reproduce the banging noise at the time (I checked their gas safe certificate was in order) and they bled all the radiators. It has got even noisier since then though. It makes louder banging noises now that I have started using the central heating as well as the water heater.
I recall that it made noises before it got the new PCB a few years ago but it also cut out then, whereas it works now but sounds dangerous and seems to have the green light flashing nearly all the time. I think the noise coincides with the pilot light flaring up and going down again - does that have anything to do with the pump? (I do switch it on once a month or so as I had a pump seize up on a previous heating system during the summer.)
My boiler is about 8 years old and the PCB is about 5 or 6 years old so it shouldn't need replacing should it? However, I know this is the rubbish boiler that was feature on Watchdog!
Many thanks for any help.