Hi, yes it will be the diverter valve but you only need to change the diaphram parts cost £20.00 very fiddly to fit. It is worth changing this if over 3years old. If after changing the diaphram and you still have hot tap water problems i e hot water going off and having to restart the boiler. Then you have a gas valve pc board fault, to cure this you wil have to ask me the question again after fitting the diaphram. This is a easyer problem to solve than the diaphram and cheaper.
Bobby Rob
January 2007