"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 !!
 Mike
 August 2010"
From this page: 
https://www.howtomendit.com/answers.php?id=171210
Lots more solutions on that page (as well as a lot of adverse comments about Peugeot)
                
 3rjbf
                
 May 2022