hi the diverter is probably stuck , is it a combi boiler or pressurised hotwater if so it will need new taps as they are probably not ok for high pressure systems , if not then you need to turn off both hot and cold supplies and apply a spanner to the central piece of the tap (where the hose is connected) and undo it , be careful as there is a spring inside free it off then refit simple as that.
steve
May 2008