You will probably find that the contact breaker switch which is part of the door locking mechanism is broken.
Go to espares.co.uk Click on their 'Advice Centre' tab to be shown what to do.
This site will have the spare parts that you need-just follow their menu for your model & serial.
Geoff, deep down inside Wales
July 2012
I can answer this question...
washing machine door fails to lock on start of wash cycle?
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix washing machines.