Replace the interlock assembly.
Spares suppliers:
www.4directelectricals.co.uk
http://www.ukwhitegoods.co.uk
www.ukapplianceparts.com
http://www.domesticspares.co.uk
http://shop.ezee-fix.co.uk
http://www.espares.co.uk
or simply go into google and type in "washing machine spare parts" - selecting "pages from uk".
If you are familiar with ebay, there are dozens of sellers running washing machine spare parts shops at very competitive prices.
Repair and service your own washing machine. Go to www.2ndwave.co.uk/manual.html
dannyboy, November 2008