I had the same problem. It would appear the most likely fault is the sheering of a plastic internal part in the top up valve (the bit which the screw piece is attached too).
Glow worm updated this piece which is now made of metal internal parts. I bought a new one for £13 and from start to finish, it took less than 10 minutes to replace. Anyone who can use a screw driver and remember to turn the water supply to the boiler off first, can do this simple job. It even came with new fixing clips and washers.
Oh and you don't even have to drain the boiler first. Just have a towel ready for any spillage and make sure the electics are covered.
Hope this answers your question.
Gary (not a plumber), September 2008
