Gary thinks it's a young sparrowhawk, and it's getting harassed by a magpie and a crow. When I read Gary's Little Cutie in the DFC, I didn't realise just HOW much the guy knows about birds. If you follow him on Twitter, half of his observations are about Davros the blue tit that lives near his balcony.
Edit: Joseph Beale thinks it's a kestrel, not a sparrowhawk. Yes?