This is most likely the thermostat located in the box section at the rear of the unit. Remove the back and there will be two thermostats. The one on the left usually with a green spot is a one shot stat that operated when the dryer gets too hot. You need to buy a replacement kit which will replace both stats for about £16 (try espares online). The green spot stat can be tested with a test meter or if it is concave in shape in needs replacing.
Dave, April 2008