Overheating light on in boiler (Potterton Flowshure Plus)?

I have a Potterton Flowshure Plus boiler. When I put the CH on today there was a slight rumble and squeaking from the Myson mpe 322 valve, so I turned off the electricity. After I switched it back on the boiler fired up, but the squealing became even louder after a few minutes and then the overheat light came on and now the boiler won't fire at all. I have a suspicion that the valve is broken, as it has gone before under different circumstances. Is there any way I can get the boiler to fire up until I can get a fitter out. It's the only means of heating and hot water, and we could all do with a warm shower. Thanks in advance for any ideas