Just coming back to say thanks again. After a bit "fun" trying to coax an ancient stopcock into moving, I moved on to opening the tap and it was indeed a knackered washer, that was almost in two pieces. Replacement in and everything's working nicely again now. (y)
Apologies if this is a very stupid question but having trouble with one tap in our kitchen.
Basically it will only open if you turn it very very slowly and will eventually open and work fine.
It seems to work slightly better once opened and can be opened/closed slightly more quickly for a while...