How can I fix my Hotpoint Aquarius TL21 tumble dryer?

My tumble dryer usually goes onto a cool cycle for around 20 mins at the end of drying clothes to prevent creases. It appears to be stuck on the cool cycle, will not switch off unless unplugged or the door is opened. As it is only blowing cold air it does not dry clothes. I am concerned about buying a new switch as I believe they cost in the region of £60 as it could turn out to be something else that is causing the problem