Hi,
You may have an airlock in the heater matrix. On the Talbot Express the heater radiator is well above the engine block height and if the engine has had its coolant drained at any point in its life you can fill up the engine block but the heater unit is still empty, and that's why the heater blows cold even after a long run.
To fix this, with the engine cold, take off the expansion tank cap and put the heater controls on hot, all the way over so the valve is fully open. Then start the engine and let it idle but keep an eye on the water level in the expansion tank. Keep topping up the tank as it needs it and keep feeling how hot the radiator top hose is getting, obviously keep your hands away from any moving pulleys etc....
You will eventually feel the radiator top hose get very hot then go a bit cooler as the thermostat opens, ( if its working ok ).
Then have a feel of the heater matrix hoses, they should both be getting hot as the coolant is pumped through and the airlock is vented through the open expansion tank. You may even see air bubbles coming up through the tank.
When you are sure there is no more air in the system go inside the van and try your heater, it should now be blowing hot. If all is ok switch off and let the engine cool before replacing the tank cap.
As long as you don't overfill the expansion tank it won't boil over as there is no pressure in the system without the cap, all you're doing is bleeding out the excess air.
I had to do this on our Talbot Express 2.5D when the bleed off pipe on my degassing bottle broke off.
Hope this is useful to you.
