If you have lost the red programe key you will have to replace the complete lock set and immobiliser because they are matched and without the red key you can not just fit a new lock set because you will have no way of re-programming the immobiliser. total cost for parts is about £170 for the immobiliser and £70 for the lock set, speak to your nearest peugeot dealer and they will help.
PAC, July 2005