worcester bosch 230 combi rsf Diverter valve?

Hi I have a worcester bosch 230 combi rsf and believe the Diverter valve is faulty. I have central heating which works fine, but no hot water. if heating is on and i call for hot water it sounds as though it is switching over but we only get cold water, also if heating is off and we call for hot water, the boiler will not fire up.
is the diverter valve easy to change etc and how much are they?
any help or info would be most appreciated.