"Right lay on seat in the fully reclined postion and push the levers back with a bit of force . You should hear it click then whilst still lying on it push levers forward quickly this worked for me try a few times if doesn't work as the mechanism is jammed and it needs to be un jammed force helps some times !!! £650 for a new seat my arse!!! Annoying problem granted hope this helps !!
From this page:
Lots more solutions on that page (as well as a lot of adverse comments about Peugeot)