What part do i need?

My boiler is a Worcester bosch 240 Combi. The CH, when switched on works fine. When i turn on the hot water taps they get hot-ish eventually, but the CH also comes on. What i do know is, that the diverter valve still activates the two micro switches, and fires up the boiler when the hot tap is turned on. it seems the micro switches are working ok.
I`ve also been told the diaphragm in the divert valve could be knackered.
Has anyone had the same problem. and what the solution is.