Richard,
Don't waste time trying to replace it with another GTD radiator - the replacement will eventually fail as well.
Instead, contact Mel Johnson at
Docking Engineering of Silverstone (01327 857164) and ask him to make you up a GT40 radiator.
It's the same size as the GTD item, but being made of aluminium weighs a fraction of the GTD and will be much more reliable when mounted on rubber bushes.
Mel normally takes 2 or 3 weeks to make one up, and will take you round the shop when you go to collect it. Some of their stuff is a work of art.
Tony