Today I repaired mine which had the same problem.
You most likely need to replace the thermostats.
If you know what you are doing, you can take off the small metal cover at the back right of the dryer. The thermostats are two at the top of the cover. You will need to take a metal bit out of the cover which has the element to get access to the thermostats. Each one has two wires going to it.
Please only do this if you have some knowledge of electricity as its up to you to do this safely once everything is unplugged from the mains! To cover myself i replaced another thermostat which is fitted to the door (above it). This one requires more dismantling but I worked it out. This is accessed from the top, so you need to take off the top cover (take off back top screws for cover and slide it backwards)
A top tip - use a digital camera to take photos as you go in case you forget which wires go where.
NB - I take no responsibility for this advice!
Gavin
October 2009