potterten profile 50e?

My profile 50e, will only come on for about 2 mins and then switch off,this happens in both modes
water and central heating, when the main thermo clicks it fires up,but sometimes I turn the knob and there is no clicking and nothing works.
if I leave it for a while it fires up but then turns off.
I have also tried to locate the overheat thermo but can't find it.any ideas please.