the thermostate sounds like it is at fault, there 2 thermostates fitted in the back above the heating element, relativly easy to replace.there are 2 different thermostates depending on the model.150/110 degree, and 130/90 degree, these comes as a kit and generaly will cost aprox £16 -£20
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix tumble dryers.