I'm a bit surprised to hear about a Kestrel taking a Redpoll. The reason that you saw the flock of Redpolls in the same tree as the Kestrel is probably because they didn't feel threatened. Kestrels usually go after small mammals or large insects, perhaps small reptiles. They usually either hover over an open field or swoop down from a perch, that's why we see them on wires and posts so commonly. I have a pair that nest about a hundred and fifty metres from my feeders and they never seem to attack the small birds. tyhat's not to say that it couldn't happen.
What you have described sounds like very typical Sharp-shinned behavour or how about a Merlin? In fact, the more I think about it the likely it seems to be a Merlin. Only about an inch longer, feeds primarily on small birds, very similar call.
Sorry to be questioning your id. but it just doesn't sound like a Kestrel. :h?: