Potterton Suprima 60 Suddenly stopped working?

I bought a house almost 3 years ago which already had a Suprima 60 already installed. About once every 4 months I have found that I needed to prese the reset button as the heating suddenly stops whilst the control is demanding heat. But tonight its stopped working a will not reset to anything other than a red light or red flashing light. The strange thing is, if I change the froststat to a higher temp it begins working and the rads all hot. (And yes I tried turning us the normal temp control etc) but of course I can't keep it on like this as it will just stay on constantly. There seems to be loads of info on PCB probs but not like this. Any advice?