Radiator not heating up after engine change -1999 Megane 1.4

Hi all,



I've just done an engine swap after the timing belt snapped on my 1999 Megane, 1.4 16 valve engine.



I've got the new engine fired up and its running, but the radiator is not getting warm at all, neither are the two big pipes supplying the rad. The small heater matrix pipes are getting hot and the heater blows hot air.

The temperature gauge hovers around mid way on the gauge

Thought it might have been the thermostat so I swapped the working one from the old engine and it made no difference. I bled the system at the screw on the thermostat housing as well...



Any ideas what might be wrong?

I have a new water pump and timing belt on order, not fitted yet, was going to fit them anyway, but what is the most likely cause of the problem?



Thanks!

David