It could just be on it's way north.Def sounds like a Ring Ouzel.I was watching the rabbits in the meadow behind my house early this morning through the binoculars and having a laugh at the ineptitude of next door's cat trying to catch on. Totally hopeless and the rabbits seemed to know it. I scanned across the field and spotted a male blackbird that appeared to have some white on its chest. It was broadside on so I watched it, hoping it would turn and face me. It did, and on its chest was a perfect white crescent shape stretching from side to side. I noticed, too, that its wing feathers were a little bit more grey in colour rather than the black one would expect from a male blackbird.
The only bird in the book that fits what I saw is a Ring Ouzel, but what is one doing hopping around in a grass meadow used as a horse paddock in Wiltshire?