FIXED!!!!!
I tried the suggestions about blanking plates for the rad to no avail. Decided to go back to basics and get the cooling system flushed through. Engine block and heater matrix flushed through separately. There’s a short hose that runs from the rear housing on the head to the heater matrix. This little hose has a spring in it in order to stop it from collapsing I guess. The spring had caused a blockage in that small hose which was easily flushed out. The car is now running at normal temperatures under load and on tick over in a queue. The fan cuts in and now the temp immediately drops unlike before. Many thanks for all of the suggestions, and the time taken by my Morgan friends to write them. Just goes to show that going back to basics sometime is the best way.