I have fixed 3 of these now, it is the thermal cut out which has gone. This is very easy to replace and is available on Ebay (cheapest) for about £20.
Take the oven out the cabinet, unscrew the top panel and simply swap over the thermal cut out, put it all back together again, turn on remembering you have to adjust the clock (don't know whay you have to do that!) and your done!
J, May 2009