Common fault of these, it's done it to me, it's not the ECU at fault it's jsut typical crappy Peugeot wiring,
Lift both the front seats up and you will see some nasty cheap orange connectors, on the wiring loom attached to the underside of the seat, You will have 2 or 3 (most likely 3 as you have a Phase 3 car) best thing to do is chop the connectors out and either solder the joins back together or crimp them, obviously make a not of how they go together as you really don't want to cross the wires,
2nd place to check is the airbag ecu which is under the dash board and to get at it the centre console has to come out, It's easy to take out so don't panic, same again you will notice some more of those orange connectors, do the same chop the buggers off and solder or crimp and also the large plug that on the right hand side just check it's plugged in properly
last place to check is the rotary swtich behind the steering wheel, now these are avaliable from your pug deal for about £30 so not expensive, romve the airbag and steering wheel, (only do this if the other 2 things fail)
if it still doesn't fix the problem then your airbag ecu might be buggered and need replacing
don't get the main ecu flashed for this as it will cure the problem for about 5 mins and you will taring your hair out cause it just cost you £50 for them to do that
also the best to disconnect the megative terminal on the battery before you do it and wait 10 mins or so just to make sure that the airbags don't go off when you are hacking at the wires