My local garage fitted a pressure cap on the radiator , Its called a recovery cap allowing the cooling water to be sucked back into the radiator. They also fitted a pressure cap on the overflow tank . The second cap in my opinion should not hold pressure. Apparently this system is used on a lot of modern cars.
My conclusion is as expressed by all replies is the blanking cap fits the radiator and the pressure cap goes on the tank.
I will need to consider further why the water does no return to the radiator.
Could be a very slight air leak .
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