Hello,
if the engine is working fine and running at a decent temperature then it's to do with your heater coolant circuit - this comes from the head and goes through the heater matrix in the dash and back to the engine bay joining up around the water pump housing. I think I'm right in saying this always flows irrespective of the thermostat opening. I would reckon you probably have your heater valve stuck closed - this is a cheap fix.
If you take off the spare wheel, and look up into the gap above you will see the heater pipes going through a rubber grommet into the cab area behind the dash - one of these has a valve on it which is controlled by a cable from the middle lever on your heater controls. You might find that the cable has become unhooked at either that end, or the lever end - you can check this by taking the radio out and facia off the heater controls. On some vans you can get to the valve by removing the gear lever gaiter.
Since it's cold a very cheap fix would be to just leave it fully on and fixing it properly when it gets a bit warmer!
If it's not this then the heater matrix may be blocked or there may be a blockage somewhere else. Someone else might have a better idea.
Good luck sorting it and happy new year!
Ewen
(P.S. this is a heater valve:
http://www.brickwerks.co.uk/shop?page=s ... ater+valve" onclick="window.open(this.href);return false;
if you need to change it you'll want the clip too)
