My Tumble Dryer keeps stopping mid cycle?

My Hotpoint TV30 Tumble Dryer only works for no more than 5 minutes at a time. Upon opening the door, the heater can be seen to be functioning as the clothes are warm. Shutting the door and switching on will start the dryer but only for a few more minutes before it stops.
The hose is not obstructed or kinked, the filter is clear and there is good airflow around the machine.

If it is a thermostat that needs replacing, which one is it (as I assume there will be more than one) and how do I find it?

I have a multimeter if there is any test that can be done.

Bill
June 2010