Why have you lot got him pissing about with his mains water, as he’s wrote it it’s when his heating is on, what your telling him is to cure water hammer( and then that’s just a quick fix and isn’t addressing the problem).
Just get some one in ’cause you’ll probably make it worse and cost yourself even more money, ’cause a lot is going to depend on what style of heating system you have(e.g. C plan, Y plan or S plan, as they all work a little differently) and if it bangs on hot water or just on heating.
Not something you can help with on an internet forum, so don’t mess I see it all the time at work when people have messed around, made it worse then they phone us, and being British Gas we ain’t the cheapest. 🙂