its just plain odd...
simply put I would reduce the mains just after the stop cock to 3 bar, with an expansion vessel and be done with it.
by having two separate valves you will end up with unbalanced hot and cold, and this will most likely cause you some problems.
Thanks hazetimesfive, I think we are having these problems as the rads are unbalanced (despite me getting him round to balance them already). I should say that they installed these two PRVs in a belated attempt to stop the noise but positioned it as an 'experiment' as they said they didnt' know what the problem was and therefore what else to try. I am most concnered that correct install of one PRV/expansion vessel or whatever is most suitable should have been done during install and would have avoided the current many pains andn leaks (shower also squeals...). Thanks so much for your help.