Well having posted about a problem for advice it's only polite to provide feedback for those who bothered to offer advice. Makes you wonder why some people bother reading and then posting in threads they say they're not interested in but I'd say it confirms something quite revealing about them...
I'm hoping that any reader of this thread with a similar problem will have a better insight into what the issue is and what can be done to solve it. It's clearly not necessary to change the entire valve assembly at quite considerable cost but how many ordinary people would know that and how many plumbers would have suggested it as a nice little earner? Maybe MrK would prefer that a little old lady somewhere gets ripped off for £200 when the real problem was a component that costs £15 and is easily changed. Bit sad that really but maybe it makes him happy in a perverse sort of way...