Sounds like a faulty switch within the interlock unit. Replace interlock.
Typical interlock replacement: Power off, remove mains plug. Remove the plastic band that holds the rubber door seal to the front cabinet. Un-hook the rubber gasket from cabinet lip on the door lock side. Put your hand in and hold the door lock whilst removing the 2 screws you can see from the front. There might be a pressure pipe attached to the lock, if so, just pull it off. Make a note of the position of the connectors and disconnect wiring. Refitting is, as they say, a reversal of the above process.
Repair and service your own washing machine. Go to www.2ndwave.co.uk/manual.html
dannyboy, February 2009