Just had the same on my 307, but on RHS, I found the weld holding the pivot pin for the height adjuster snapped off seat frame.
Had to disconnect battery, remove centre console disconnect airbag control and cables under seat before removing seat.
i have cleaned up weld area ready for welding back on but as there is an airbag in the seat I need to check first with peugeot if electric welding is likely to set airbag off.
GL
HTH
ejr
May 2010