Combi Boiler - Heating but no hot water (Vokera Excell 80 SP)?

Hi,

The above boiler was fitted in my house when I moved in. The problem was that the hot water temperature adjuster had been snapped of the PCB by the previous owners or someone they had paid to "fix" it. This meant that the heating worked fine but the hot water would only run for a 30 seconds before running cold.

Yesterday I replaced the PCB and expected this to solve the problem but still only the heating is working. There seems to be another problem. I can only assume that since I now know that the temperature selection is OK that there must be something wrong with the signal that tells the unit that the hot tap is drawing from the boiler and to therefore heat the water. The water temperature guage is always sitting on the very minimum.

I have read the the diaphragm may be damaged but reading my service manual it seems that there are two (domestic and flow). Which one of these should I replace and also is there anything else that I should consider in this problem?

Thanks in advance for any repsonses.