Heating/Hot Water Problems with Ravenheat RSF100ET?

Hi
Original problem was not very hot DHW, but burner running flat out.
Changed both DHW and CH thermistors, DHW temperature improved, but boiler still running flat out, even on DHW low flow rate (no flame modulation).
Then I noticed some of the radiators were getting hot, which explained why the boiler was running flat out (this must have been happening at the beginning).
Replaced the diaphram assembly in the diverter valve, no difference.
Replaced the microswitch assembly, no difference.
I can get DHW whether the switch is set to only DHW or DHW/CH, as it should do, but when supplying DHW on the DHW only setting, it is still running flat out and heating radiators.
When set on only DHW and you turn a tap off, the pump runs and the boiler fires up flat out and starts heating the radiators.
When set on either DHW or DHW/CH, the boiler fires up flat out and without any DHW being run, you can modulate the flames with the DHW dial, but not the CH dial.
The pin from the diverter valve is definately moving and operating the microswitch.
I am assuming the diaphram is fitted correctly i.e. there is a small spring on the diaphram assembly and it is this end that fits inside the separate large spring?

I hope that makes sense!
Thanks for any help you can give.