Possibly the problem may be caused by a worn mixer tap, either on your kitchen sink, or more likely a worn shower mixer. Although Brit Gas should have told you this. Cold water is crossing over to the hot supply via the worn mixer possibly. Causing many problems.You can test by dismanteling your shower and blocking them with pushfit stoppers, although it is the shower with the problem, but maybe you will notice other probs not mentioned here are OK now. hope this helps.
Jex
August 2008