Thanks all.
That particular bird had been in and around our garden a fair bit that day. And almost unbelievably when I took those, was only about two foot above our cat. If I didn't know better I'd say it was teasing him.
Which is a brave move given the amount of animals he brings us.
That said, about a week ago I did notice a couple of birds - couldn't really see what they were, but blackbird size - keep landing either side of him in turn. That is, one would land, he'd start to chase it and it would fly up, then the other one would land the other side of him and he'd chase that. Repeat, repeat. Went on for a couple of minutes.