Immobiliser codes?

I too have a Peugeot 306 DT 1997 My handbook was taken when I left it on a counter at Halfords so I need to know how to change my code having used it for about the last 9 years
I have asked at several garages to no avail surely someone must know!! I have accosted several owners of 306's but they do nt have an immobiliser I know it is something to do with the S pad and entering old code then new code

Please help