306 Turbo diesel heater blowing cold

my 306 turbo diesel has had the heater matrix changed, the electrics tested, it has had a new thurmostat and there is no air block in the system. the temp gets to 1/2 between 90 degrees but it is still blowing cold air through into the car, however it does blow hot air only through the 2 middle grates, i have turned these off and let it blow only through to the window on full power but after