Thanks to those who replied to my post. I had a plumber out who told me there was a leak between the external stopcock on the pavement and the internal one under the kitchen sink. Of course I had visions of water under the floorboards etc. Now the local water authority have just been for the second time, dug out the external stopcock, found a definite hole in the pipe nearby.. fixed, new tap and all sorted. Just a mystery what caused such a definite sudden hole in the pipe. I learned a lot in the last week or so about how it all works and if I was ever doing up another old house I'd definitely look into putting the newer plastic supply pipe in to replace the old pipes because sooner or later they are bound to give trouble the length of time they have been in place. The water board have a backlog of leaks and cannot keep up with problems springing up all over the place in (mostly) older houses. I was very lucky they could prioritise my job. I hope my experience helps someone else. Don't delay in your ponderings about what the problem is, get the experts in straight away.