No hot water [Valliant boiler, Turbomax VUW 242]?

Just recently my boiler won't heat the water in the bathroom taps or shower, the boiler fires up but the water is just slightly warmer than the cold tap. Hot water is fine at the kitchen sink. Today I played around with it a bit and noticed that the central heating flow pipe gets warm while any hot water tap is turned on (heating off). Looking at the manual I think this may be the divertor valve, but after speaking to valliant they think it may be the secondary heat exchanger.
They didn't give any explanation as to why either part would still allow hot water at the kitchen sink but no wher else.
Needless to say I am now confused and wonder if someone can advise. Following the manual I would be confident to replace either part but need to make the right call.