1994 Favorit GLXiE, heater will only blow warm on the "screen" position. Plenty of (cold) air comes out of the floor vents. I've tried back flushing the matrix, and the valve appears to by working correctly. Anyone got any ideas, before I get frostbite?!?!?!

If you're getting hot air out of the heater then the matrix and valve must be OK. I would suggest checking the air distribution control, the operating cables and levers on the body of the heater unit - one of these may be broke or dislodged.

Is thermostat fitted and of correct type?

I seem to remember my 1990 Favorits were not too bad in the winter at warming up. dont remember too much coming out of the floor vents when switched to screen, I seem to remember it blasting out of the face vents - I think always left it in this position in winter. And I think the Fan worked faster than the heat available. Yours is a late Favorit so may have had changes over my early 1990 ones

The engine seems to warm up quickly enough, so I assume the thermostat is OK. It seems as though the incoming air doesn't pass through the matrix when the heater is on the "floor" position. The only othe thing is the matrix seems a bit loose in the housing, so maybe air is flowing round it rather through it. At least the heater is easy to get at, not like on these new-fangled modern cars.....