Methinks the first tap that you refer to is actually jammed and not rotating - or its totally jammed up with limescale. I think you have really got to go back to the beginning and renew the tap that is on the hot feed to the sink - easy if there is enough 15mm pipe remaining after the old one is cut off, and if its easy to shut off the hot water supply to the sink. Easy to give advice sitting in front of a laptop though.....best of luck with it.
John
January 2008