All that is happening here is that your surveyor is covering their rear-end because they are not warranting that the service installations work. It depends on what you asked them to survey.
Its worth asking the seller for evidence of when the last service was done on the gCH - and if the system is old, then paying for someone to look at it, if you don't feel competent to have a general look at it yourself. I say this because it could be expensive to replace.
Unless the rest of the house is older than say 25 years, I'd just go around checking that everyone else works. And I mean everything - every hot and cold tap, every electrical socket and switch, open every opening window, check every locking door, flush every loo. You'd be surprised how often some people can't be bothered to fix stuff.