That's typically a bad door switch.
If you or a friend decide to work on it, we have *critical* safety and disassembly info at our site, as well as other specific troubleshooting information and help.
In my professional opinion, people should not work on a microwave unless they are fully informed and aware of the deadly hazards and necessary precautions.
You can find safety and disassembly text files (as well as a text file which helps in diagnosing and repairing door switch problems) at http://www.microwavedisplay.com
In the US, you can order parts for Sharp models from Tritronics at http://www.wehaveparts.com.
You can find wiring diagrams and parts lists and order parts for most other US brands from the Pros here: http://tinyurl.com/gv383.
You can find UK part suppliers and authorized servicers via http://www.sharp.co.uk
William Miller † MicrowaveDisplay.com, March 2007
