Blimey tony that's harsh mate, I only chemical flush it's fine and is quoted on benchmark that a chemical flush complies, he may also be right re the shower mixer mate! Especially if it's been converted from tank an cylinder system, it might have a low pressure shower that's passing, or as has been said it may be piped reverse... Isolate the cold water inlet to the boiler an open any hot tap if u got water flowing it's probably the shower
The point is he said he would do a power flush, not a chemical clean. The shower unit is about a year old, and though not impossible, it was very unlikely that this was the problem, and indeed, it turned out to be the case. And indeed, he should have isolated the shower before suggesting another £170-00 was spent on a part that was not needed.