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I've been studying the books again and the best contender seems to be a sparrow-hawk but none have a distinctive yellow throat. What I think may have happened (slight embarrassment here!) I was looking out from a first-floor window and he appeared with a great flourish from the direction of the obscured bird table below. He plonked down firmly on the lawn with his back to me - that's when I thought he was a pigeon- I didn't have my distance specs on, and when he turned his head I saw the distinctive 'yellow throat' then when he took off I saw the yellow legs, but I now think that yellow throat might have been an unfortunate blue tit he had just nabbed for his breakfast!
I'll keep my eyes open in case he was an extremely rare bird indeed, and thanks to all. :0)