Said to someone who, out of character, is behaving in a manner that could be considered a caricature of some other person (famous or not). Eg, if someone was spouting pseudo socialist rhetoric, you would say: "You are Karl Marx, and I claim my five pounds." Aparently, there was a newspaper in the seventies, some say the Daily Mirror, that ran a contest whereby a photo of a person would be printed in the paper, together with their whereabouts (usually some beach or other), and you would say this to them, winning your five pounds. The phrase was later picked up by Your Sinclair magazine as an occasional response to readers' letters.
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