Call a Master Locksmith.
To find a local one, go to http://www.locksmiths.co.uk/
the search is on the front page.
If you have lost the key, safest option is to replace the lock cylinder. It would be prudent to choose one that is snap-resistant and bump-resistant.
R Phillips
December 2013
Automatic Answer
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howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix windows and doors.