I had identical symptoms. It was fixed by removing the bottom grey plate at the rear of the drier. The larger top panel does not need to be removed but only loosened at the bottom. This gives easy access to the capacitor that is attached to the motor by a single nut. A replacement capacitor was obtained for around £10 from a local appliance repair shop. This was an equivalent of the Bosch capacitor. Fitting this cured the problem. Make sure you take note of how the four spade connectors are attached to the capacitor.
Anon2, April 2009
Most likely the problem is a faulty capacitor attached to the motor not giving a sufficient kick to get it turning. Bosch website costs this at £43.