Well,.....perhaps the chap who suggests tapping things with screwdrivers, knows exactly what he's talking about, and is absolutely correct,.....but,...do we really want to start fiddling with boiler internals if we aren't a trained and qualified heating engineer ?.
Apart from the fact that doing so is probably illegal, it might well also prove to be unsafe, and potentially very dangerous, and or, expensive, and personally speaking, the idea of messing around with boilers fills me with horror.
My suggestion would be this, contact Worcester Bosch, yes, I know that might sound obvious, but it doesn't necessarily occur to everyone as a first option, Yellow pages might be the first thought of many people.
http://www.worcester-bosch.co.uk/homeowner/customer-service/contact-centre
http://www.worcester-bosch.co.uk/homeowner/our-company/head-office-contacts
Why contact Worcester Bosch ?, well, we've probably all had the experience of having to call in an, " expert ", of some sort to fix a problem, and then finding that they hadn't a clue, although they do know how to figure out a fee, which also usually includes a callout charge.
The thing about Worcester Bosch, ( and this may well also apply to other boiler companies, I can't say for sure), is that their technical service people are very helpful, and they really do know what they're doing and talking about, as do any local engineers that they recommend.
They can send one of their own service engineers to do repairs, although I imagine that a qualified, Worcester Bosch trained, local engineer would probably be a cheaper option.
They will recommend several local installers if you are getting a new system fitted, and those installers will have trained at Worcester Bosch, on that particular model of boiler.
I don't know if they will recommend a local engineer for service/repairs, but if not, you could always pretend that you are interested in getting a new boiler installed, and mention your own model, just make sure that some of the recommeded engineers are self employed, or small firms, because bigger outfits are sometimes only interested in installing complete new systems, and won't want to do small repair jobs.
Yes, I know that we all hate to pay repair bills, but at least that isn't so painful if the person we pay really is an expert and does a good job, and there are some things that just shouldn't be messed with except by an expert, gas and electrical equipment surely must come high on that list, surgery and stripping car engines also spring to mind.
Get an expert and get peace of mind, good luck.