Hi
I have a Worcester combi boiler which has a domestic hot water problem.

The central heating runs fine but the boiler no longer fires up when a
hot water tap is opened.

However, if the boiler is already running for the heating, opening a
hot water tap will get you hot water. At the moment the only way to
take a shower is to fire up the CH, jump in the shower and finish
quickly (i.e. before the boiler reaches the CH cutoff temperature and
shuts off the gas fire)!

Any advice on what might be causing this would be very much
appreciated.
Regards
Herminio