you need to find the valve in your airing cupboard that supplies the hot water from the tank. If you have a combination boiler that supplies direct hot water from the boiler there is a hot water flow valve under the boiler.
once the water has stopped you need to take the handle or knob off the tap either by a small screw on the side or by prising the top off the knob and removing the screw under there.
you should then be able to undo the tap body, the bit between the tap and the handle or if its a knob type you should see a brass nut just under whwer the handle or knob was.
loosen the nut just a but and hopefully the tap will turn again.
Danny, March 2008
