Alpha CB24 boiler - No hot water no matter what?

For some time now I can get hot water providing the heating is turned off i.e. the boiler (alpha cb24) is turned to water only. Once I turn the boiler switch to provide heating and hot water I get heating only. There was a time when if I continued to turn up my heating thermostat in the house not the boiler itself, the hot water would eventually come through, but now it is irrelevant how high the thermostat is turned up, the water remains cold permanently. I just
cannot afford a plumber so need to know what parts I should look at changing so I can order the relevant bits. Also where's the best place to buy from please?