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There should be two huge rubber bungs that push-fit into the radtop panel, the holes in the middle of which go over the two prongs sticking up from the radiator. maybe these are missing?
Number 11 in this diagram. Two at the bottom and two at the top
You can see one of the Rubber Bungs in this Photo, which fit into holes in the Top Closing Panel, that go over the Top Pegs on the Rad to hold it in place. (Just like Greg said)
On the Top Closing Panel is a Rubber Bung each end that go over the pegs in the Top of the Rad to hold it in place