I saw a bird flying very fast over the hedgetops, making constant minor deviations in its flight path - almost kind of slightly zig zagging as it flew along. It did not have any noticeable undulating flight movement. It was making an alarm call like sound. It was a tiny bit like when you hear oystercatchers, but the sound was quite a lot deeper, and had a tiny bit of croak to it. It also sounded about once to twice per second. The bird was all black - at least, that's all that I could make out. It was like a slightly larger than a blackbird, and seemed to have a stubbier tail, and stouter body, than a blackbird. Perhaps the wings were also not quite as long too. Although it was a little bit like a blackbird, I'm 100% sure it wasn't! It kind of very vaguelly reminded me of the rough outline shape of a nuthatch somehow. Sorry for the vagueness, but could this possibly be a ring ouzel? I checked the rspb sound clip for this bird, link below, and it did not sound like this. Thanks a lot for any thoughts on the matter. (I couldn't see any colour on the rump. It did appear to be all black in colour.)
A stab in the dark here but a lot of what you mention makes me think of a Great Spotted Woodpecker.
Rough outline of a Nuthatch - Nuthatches look like little woodpeckers.
The size would be smaller than what you saw - smaller than a blackbird - but as you know, in flight sizing a bird can be quite difficult.
The call of the bird. Go onto Xeno and see if you can hear something similar under Great Spotted Woodpecker - I think there are similar calls to the one you state.
The zig-zagging of the bird in flight - I've seen woodpeckers fly like that (very quickly and hap-haphazardly).
Tail can look stubby in flight.
You mentioned Ring Ouzel, and there's a bit of similar patterning too - even though you've discounted that bird.
Lastly, here's a photo of one I caught in flight - had to be quick - and even though the black and white is quite apparent, depending on the angle of the incoming light and the available light too , the bird can appear dark in colour.
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