thanks to Peter Maddison for interesting comment. The first theory seems more likely as it first flew onto the perch from the Deben river direction. This is directly the opposite direction to the 'normal' boundary of the hunting territory of the Marsh Harrier at the small Mill River
Peter Maddison
26-Jan-2017 18:01
It might not have been an involuntary plunge - in this cold weather the bird will be desperate for food and might have attacked a wader or small duck (teal?), unsuccessfully! It looks okay when perched, and it did fly off. Perhaps just soggy, not injured. It's possible that a Marsh Harrier went for it, seeing the kestrel as a hunter competitor.