Thanks all!
Our microwave is the most complicated bit of kit we've ever had - it was originally Mrs Growster's Mum's, and when we all went to choose it, we picked up the wrong one before we realised when we got home, so it's our fault really!
The instructions are just dire - it's a Belling Super Apollo Spaceship with matching armaments, atomic reactor, fast murder and automatic hospitalisation procedures, and we have read the instructions about three hundred times, and they get worse!
The water-in-a-cup did start to work, but not as much as we wanted (superheated to 3,000 degrees Celsius), so we'll give it a bit longer next time!