Is there enough coolant in the system have you tried bleeding any air out of the system Do the 2 pipes to heater stay hot i would try fitting a new thermostat, also had cases where heater matrix is blocked with engine hot you will notice one of the pipes is cold to it,when heater fan is on max for few mins if it is cold the heater matrix is blocked and its a big job as dash board needs to be removed
Peugeot master technician, December 2008
