drying timer goes down too quick and doesn't dry clothes?

I have a Zanussi zwd1681w washer dryer. in the drying cycle the timer goes down far too quickly (30min drying cycle takes just 5 min) no other function is affected. the clothes don't get warm at all. I have cleaned out the huge plastic container in the back of the machine and also tested the heating element for conductivity and got a possitive result. i have also tested the thermostat at the front of the heating element housing and found this to work.
please could you suggest any other possible causes?
thanks.