Sime 80?

My Sime Format 80 was not giving any hot water but the heating was OK.
I checked the diaphragm and it was damaged. I have replaced the diaphragm but still no hot water. The diverter valve plunger is not moving when the hot water tap is opened. But if I force the plunger open we get hot water?.
1. Is the diverter valve also faulty?
2. If the diverter valve is faulty do I need to change the entire valve or do you get repair kits (springs, seals etc).
3. What other parts would it be advisable to change at the same time.
4. Another problem I have had for some time is that the hot water does not get very hot or it is erratic. The pressure switch fluctuates. Could the above components have been gradually causing this over a long period or is it another problem.
5. Apart from the hot water temperature dial on the main panel is there an internal setting to control the hot water.

Any help or advice

George