the master cut out i beleive is on the top of the oven screwed next to the top fan, however i think that this cutting out is covering another fault and is not the problem and is only protecting the oven from literally catching fire. if you look in around about the came place on top you will see a wiring loom with a connection that looks like a blue bullet. On an oven i worked on this was charred on inspection it had 4 wires one to the top fan one to the centre lcd display and 2 two to the left hand oven selector. I think that the blue wire had burnt off and was not letting the top fan circulate hot air thus setting the over temperature switch off just like the one you would have ontop of a combi boiler hope this is of help to you, however if you are unsure please call in an appliance engineer or a qualified electrician....
Mathieu lewis
December 2007