Spike - if none of the above work well enough - take a clean cloth or sponge and dip it in a mild solution of dishwashing (NOT dishwasher) detergent and wipe the whole inside with it - don't forget the inside top and the inside door. Then rinse the cloth/sponge and dip it in a lemon juice/vinegar and water solution and wipe the whole inside of the microwave.
BBWCHATT
The old lady in Chattanooga, Tennessee, USA
Adding a tbs or 2 of vinegar to a small bowl of boiling water would probably work. I'd keep the door open bcos of the steam, maybe closing it after it's cooled down a bit. Use the same principal to descale your kettle.
Apparently using bicarbonate of soda instead of vinegar helps to clean things, vinegar or lemon will help more to eliminate smells.