Water Temperature

Dear All
Firstly just to let you know I have a Worcester Bosch 240 combi boiler. Recently I have been experiencing reduced hot water temperature and also noticed that the radiators were getting warm when I used the hot water, even if the heating was off. I put this down to the diverter valve in the boiler and promptly replaced it. However this did not solve it. So as a test I shut off the central heating flow pipe then ran the hot water, and hey the water was at full temperature again.
I am at a loss as to what I am missing here, any ideas would be much appreciated.
Thanks in advance.
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