You will need the four digit transponder code (the transponder is a micrchip in your key which 'talks' to your dashboard, allowing it permission to start your car and disarm the security systems). This is usually found in your car handbook pack on a credit card sized piece of card or plastic.
If you don't have the key transponder code you can obtain this through a Peugeot dealer. You will need to show them the car logbook or give them the car chassis number (usually found underneath a flap of floor carpet on the right hand side of the drivers seat, stamped into the metal floor). There is a charge for this, anything from �30 upwards.
The dealer can order you in a new key. By all means try an independent locksmith as previously mentioned but I haven't heard of someone doing this before because you will need a duplicate transponder chip in your key.