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Have any other items on the car stopped working, such as wipers, lights heater etc?
Do you have a spare key?
The program is with the immobiliser, which wont show up on the diagnostics as a fault unless you have a peugeot diagnostics system where you can scrutinise it.
The most common cause of immobiliser failures in these is the transponder reciever on the ignition barrel, it takes about 20 minutes to change, and costs about £18 for the part from the main agents, so it is worth a try first off, also check the wiring to this.
If this dosent work, the next thing to do is to try re-programming the immobiliser with a suitable diagnostic computer, if you garage dosent have one, then contact an auto locksmiths or auto electrical specialist ewho will have the equipment.
Hope this helps, if you need more info, please let me know.