Tom, Steve
Thanks for the replies,the more I think about the arrangement on "our" vehicle the more I believe it is not the standard arrangement. We uncovered an old "user handbook" in the Museum's archives which describes the cooling system and water pump as
"COOLING. Water jackets are cast with the cylinders round cylinders and valve ports. Water is pumped from the bottom of radiator to a pipe with two outlets, one into each pair of cylinders, by centrifugal pump, driven off magneto shaft by chain. "
"THE WATER PUMP is of the centrifugal type, and is bolted on to the top of the timing gear case, and is driven by two sprocket wheels, one on the end of the pump spindle, and the other on the magneto driving shaft, these wheels being connected by Renold's roller chain."
I can only assume that the design changed at some time. The photo below shows a different drive arrangement – which sounds as though it is the same as yours Tom.
There are 4 unused bolts on the vehicle we are working on so is it possible it should have used an older (or newer?) chain drive to the pump maybe compatible with a different timing cover arrangement?
"Our" engine, note the 4 bolts which are not included on the earlier photo
Ian C