After a chat about it with another owner (Kerry) he pointed out that because of the basic design of the front that a high percentage of the air coming through the grille was going around the radiator rather than through it.
This blocks off 90% of the open area and should dramatically increase the air flow through the radiator. Made from plywood and held together and secured with Meccano !! (I knew that old set would come in useful sometime)
A quick 6 mile run showed that she does run appreciably cooler, with the needle between the 'r' and 'm' of 'Normal' now and fairly close to the ideal 12 o.clock position. I need to do a long run on a really hot day to really confirm it has improved the system but I am quietly confident. It certainly can't do any harm.