Hi Paul, Thanks for the interesting post. It sounds from your behavioural description that it was a Sparrowhawk.
Whenever I've seen them they are carrying out an ambush of a small garden bird.
Once I was cycling along admiring a Robin sitting in the hedgerow singing away, when suddenly, swoop, a Sparrowhawk swept down and snatched the poor thing!
Have you thought about starting a blog? Look under "Blogs" if you are interested.
