Thanks for all the replies so far - much appreciated.
So UU have now been and replaced the stop cock in the street...but there's no difference in the situation The water pressure in the bathroom sink tap is still very low (I just tested...28 seconds to fill a 1 litre bottle with the cold tap full on).
The bathroom fitter is now saying that he thinks it's the taps that are the problem and doesn't want to spend any more time investigating and doesn't think that "messing around with the pipes" will yield any benefit. The trouble is, I know rock all about plumbing so can't really speak with any authority at all...but to my simple mind, if the old pipes/taps had OK pressure and the new pipes/tap don't, that would indicate an issue somewhere in the new pipes/tap! That's not so unreasonable is it?
Is there anything in the possibility that because the old taps were individual (i.e. a hot tap and a cold tap) and the new tap is a mixer, it could be something to do with the mixer tap itself? From a quick Google it looks like mixer taps do need greater pressure to operate, but would the difference be that noticeable?
As the floor in the bathroom is now all laid, it's only possible to get at the pipework through the kitchen ceiling below...and as that's being replaced at the moment, it's an opportunity to get at the pipes. The fitter (who's also doing the ceiling!) is pushing to get the ceiling all done and plastered...but once that's done, if there is a problem that requires access to the pipes to resolve, it will either mean ripping up the new bathroom floor or ripping down the new kitchen ceiling...neither of which I'd like to do if at all avoidable!
Could you take some photos and upload them here. Best view of any plumbing we're likely to get is of the connections under the basin so take a couple of shots of that area.
Also, if the ceiling below is open take some of that too.
To test pressure/flow of pipework I would be looking to tap into the exposed pipework in the ceiling, at the closest point it enters the bathroom (best at the point where new pipework was joined to existing) and divert this temporarily into a bucket or outside using lengths of hep/Speedfit with some push fits and a couple of lever valves. This would show if restriction is in the new carcassing/brassware.
I may be wrong but from what I've read here, your fitter seems to be more of a builder than a plumber.. is the new pipework copper or plastic?
Whilst you have the opportunity (with the ceiling exposed) why don't you bite the bullet and get a plumber out to investigate for you?
I'd want this remedied before the lid below is closed up!