Bosch Classixx No Heat?

I have a Bosch Classixx Tumble Dryer WTA3002GB/01
Everything works fine however there is no heat. The empty filter light comes on during the heat cycle. I have even removed the filter but this light comes on. At the end of the cycle during the last 10 minutes (cold air) The empty filter lamp goes out. There is NO reset switch on this model as far as I can see. I have replaced the heat limiter sensor thinking this was the problem and the problem persists. I called Bosch service who wants to send an engineer out at £85 before he arrives. Any help appreciated, tech spec or workshop repair manual or anything.
Many thanks