Hot water is not so hot any more on my Worcester 24cdi?

Our Worcester 24cdi was not used for about 3 weeks while we were away. Now the hot water doesn't get as hot as it used to on the same setting, I can increase the temperature by turning the "tap" dial on the front. But this shouldn't be necessary! What could be wrong, perhaps somethings got stuck or needs cleaning, does the pressure gauge for the radiator circuit have any influence, what pressure should it be at when there is no load on the circuit? Do you top it up when it is unload or idle? Any advice appreciated, thanks. Simon.