Thanks for your response Brian. There is some compression (maybe half of what it was previously). I am assuming that the pressure was pumping faster than the overflow vent could cope with, but this is purely guesswork. I am aware that the rad is not original, it is a replacement made during the late 1940’s I believe, & as such it’s part of the bikes history so I would prefer to stick with it if it can be saved.
I will contact Graham but understood his radiators are of the Honeycomb type rather than Film?
Will update you as & when I know more. I drained the Crankcases of around half a pint of coolant this evening so need to press on with all haste.
Ps People have asked me why I use the Fernyhough moniker. This was my mothers maiden name. She was a niece of the great Eric Fernihough (the spelling within the wider family differs to this day, even through the bloodline is direct.