There was a Sparrowhawk looking for pigeons in our garden this morning. There was one yesterday that got one, and it made a real mess, so luckily it flew after one in someone else's garden this morning.
They're getting quite common in London according to the RSPB.
It's a bit bigger than I would have thought, and pretty fearless. A Simpsons-style approach to pigeon management – flooding the city with Sparrowhawks to control the Pigeon population might not be ideal.