Exactly the same problem! Must be a fault with this model! First find a thermostat, since they are obsolete! A net search of 45 pages turned one up in somerset. Electric bloke wanted to charge £200 for part and fixing. Part is £45. We've just done it. Pull cooker out, 6 screws or so fix it to unit. Put it on a table, unscrew top n back. Wires pull off old control. White control knob pulls off front to show 2 allen screws fixing the control. Whether it now works, time will tell! Otherwise up to £800 for a new cooker. Never AEG again. Good luck.
PCS, December 2009