Thanks for the link - it sounded very much like my problem (even down to working on it in the cold) but I don't believe it is the heater control valve or the heater.

I did get a chuckle from the guy saying he hoped it was the valve cover gaskets and then later, suggesting that maybe if it was the heater core, turning the control valve off would isolate the problem.

If you come up with anything else please let me know, it thought it was a ringer - I will look at the heater issues again.  Gary