Hi Paul,
You mention that hot air is being blown through the radiator...............so...is this Reverse Flow Cooling on your Saracen? If it is, then air from engine compartment is being drawn through the "beehives" on the top of the engine deck and blown out the front. Now, if it is like this, then have you got the sheetmetal and rubber shroud on the front of the vehicle? If not, then this is why it is getting hot. Forward motion is keeping the hot air in the engine compartment with out a shroud on. Have come across this several times.
Forgive me if I have prejudged your vehicle and got it wrong, but this sound like the problem.
Richard