Hi Robin, in nearly all microwaves domestic and commercial the lamp and motor circuits are common. I don't know the model you refer to but, the failure is of a common part. look for either a fuse, unlikely, a blown piece of pcb track, or a failed relay (unlikely)
For any further assistance parts etc contact me via microwave-repairs.co.uk. The rules of this site prohibit me from giving an email address in a reply.
patrick
October 2009