Just to add to the answers here, in a little more detail.
If your oven is cutting out after 20 - 40 minutes of use, it MAY be the thermal cut out that has burned out. This is a part that is quite easy to buy on the internet, and is fairly easy to replace. (Cost my £12 to buy a replacement)
The thermal cut out is a rubber bulb about an inch long with four wires, two white and two blue, that have push on spade connectors on the end.
TURN OFF YOUR OVEN POWER AT THE MAINS, ONLY DO THIS IF YOU ARE COMPETENT AND SAFE, THIS INFO IS HERE SO YOU KNOW WHERE THE CUT OUT IS, IT IS YOUR RESPONSIBILITY TO BE SAFE.
To install, remove your oven from the enclosure, this usually means opening the oven door, and about two inches from the bottom of the control panel there will be a hole on each side of the opening, most often with a rubber grommet. There is a screw in each of these holes that hold the oven in the frame. Carefully remove them and gently pull the oven out. Make sure you have somewhere to put the oven when it is out of the enclosure.
On the top of the oven, at the front, there is a removable panel, mine was held in by two small phillips screws. Remove the screws and this panel, and you will see the wiring for the oven controls, and amongst those wire the thermal cut out.
Almost everything I have read and seen tells me that if it is your thermal cut out that is the fault, the bulb will be black, and in fact mine had left a scorch mark on the metal panel.
To replace, carefully ease one of the connector of the old cut out off, and immediately replace with one of the connectors or the new cut out. (blue for blue, white for white). There are plenty of spare connections and bits of wire in these panels, so replacing one wire at a time will make sure you don't forget where the cut out was connected.
Once all four wire are replaced (it does not seem to matter which blue wire you choose to replace which, or which white you choose to replace each white, as long as the are the same colour), replace all of the panels & screws and you are done.
Hopefully you now have a working oven, for about £100 less than if you called someone out.
Good luck, be safe.
MooseMan, November 2013