In my experience this was one of the 2 thermostats that are in the top part of the heating element on the back of the unit. It seems that Hotpoint and Zanussi dryers have the same thermostats so many suppliers can get hold of them. Mine came from espares and were about £15.
After fitting them it all worked fine..... then the motor gave up. If you need to get an engineer in to do this (at £80 an hour) buy a new machine.
Neil Dawson, September 2007