Halstead Boiler - CH working but no hot water?

Hi,

I've got a Halstead Quattro Gold combi boiler, the other day the hot water stopped working but the central heating is still fine. The water does warm ever so slightly but would describe it as luke warm at very best.

Now looking through some of the threads on here, people have suggested the diaphragm on the diverter valve. A plumber friend has had a quick look and shown me that the pin is actually rising to meet the diverter switch (so unlikely to be the diaphragm). He's a bit stumped as he hasn't had much experience with Halsteads.

Does anyone have any other suggestions as to what may be causing the problem?

Thank you!