when you put the hot water on the centeral heating also comes on ?

First thing you need to check that the motorised valve is working correctly.
does the pipe that feeds the radiators from the diverter valve or two port get hot when the hot water is on.
if it does then you need to either replace it or take the head off the valve and try to unstick it.

Brian Emian Plumbing & Heating Norfolk
April 2011