I had the same problem recently, and if you pull it out, on the back there is a round cover that you can unscrew and underneath are the thermostats, they look like oversized watch batteries, if you take them to an electrical shop they can test them for you and see if they need replacing. If they do, they are only £12 to replace. If its not the thermostat then it may be another problem. But it worked for us, and now it works like new.
John
February 2007