Ok - so not quite an answer - more of an update to save any well intentioned folk out there wasting their time ..
stripped the thing down and not only is the thermal cut-out blown but the motor itself is burnt out. The motor itself was full of dust despite having religiously followed the guidance around filters and spent £30 only recently changing them .... When I phoned hoover for advice they suggested that it was due to using the hoover in a very dusty environment and so I am now heading out to buy 2 new vacuum cleaners - one for removing the dust to protect the other one from similar abusive use ...... (suspect they will not be of the same brand) .... ;-)
Simon
March 2007