I have just had this problem with my dryer. It would only work if the on button was held in. Using the info and advice on this site, I have just replaced the PCB and now have a good working dryer again. Total cost £15.74 ( Part £10.75 post £4.99 plus Vat from 4silverhillspares.co.uk). Thanks everyone, especially Dave and his excellent instructions.
Happy New Year.
Gerard, January 2007