Worcester 9.24 Electronic RSF D.H.W. problem?

Hi.
My Combi boiler (Worcester 9.24 Electronic RSF) has developed a problem with the hot domestic hot water system. When a hot tap is turned on anywhere in the house the flow rate is very much reduced and the water is extreamley hot. To run a bath it takes about 50 minutes and the bathroom is full of steam. This problem has only just started. Has anyone got any ideas as to what's wrong?
Cheers Ian.