Yea, I think you are right that I was too quick to pull the heads. I wish I had tried to run it without the thermostat because everything is pointing that way.
I've checked the radiator itself and it seems clear. I'm convinced that the water pump is working because the cab heater was pushing out a lot of heat. As far as I can tell the cab heat system bypasses the thermostat.

I had put water in the overflow bottle thinking it was an expansion tank - but of course it's not because the radiator cap cannot suck water back out of it. But the effect of doing that was that i was able to hear that it was pushing out steam when I switched it off. For all I know it may have been doing this for a long time but I had just not noticed. As I said before when I heard the steam coming out I assumed the radiator cap was failing, hence the attempt to "help it out" during the long trip ( a mistake).

Thanks very much for the replies. Any information you can give me about the cooling system would be apreciated because this is the first time I've had this engine apart and I only have the Haynes manual to go on.

Thanks
Laurie