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Thank you Rowanwitch for the link, I used it and as far as possible answered questions, but they were not all applicable, for the bird I saw (through my binoculars) was mid-brown above and from what I could see of the back half of the bird underneath it was a beautiful fluffy pure white, smallish head ,crook beak, and not that much bigger (standing with wings closed ) than a wood pigeon, It hopped further up into tree and was obscured from view when it flew off. RSPB site likened it to a Kestrel, but this certainly didnt have stripes on its underparts. Haven't seen it since,but it will no doubt be back again, it hasn't quite finished off all the collared doves yet!