Hi Andrew, first off if the coupling is faulty it will be blindingly obvious. if the motor has failed it needs to be replaced. To test the motor you need to cut open the bottom of the oven and remove the motor. Apply 240volts and test to see if the drive shaft rotates. Contact me via www.microwave-repairs.co.uk or 01424 447799 for step by step help
kindest regards
patrick, October 2009