faulty oven?

Sounds like the thermostat packed up
Go to: www.espares.co.uk for your spares and advice BUT before you do try this. Pull off the thermostat knob of its shaft and light the oven. Now, using a pair of pliers turn the thermostat shaft to see if it makes any difference. Sometimes the build up of grease behind the knob allows it to slip round the shaft without it actually turning. Just a thought.

GB in Wales
January 2013