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Have you tried removal of all four screws and then lifting from the back to a 60deg angle?
Hello George. Many thanks for this. By the time I received it, I had managed to remove the top. I think it had been held down by some gunge that had dripped on it, and after a day exposed to sunlight this may have dried out; in any event, it suddenly responded to my lifting. Thank you nevertheless for your answer, which was on the right lines.