At the time, we had no idea what was going on - we supposed it was an interloper. However, we've now discovered it was most likely a 'rook parliament'. Legend has it that one of their number is 'put on trial' and if found guilty, put to death. In reality, it's a way of dealing with sick and ailing rooks. Whatever the reason for it, the outcome is generally the same - death to the individual. In this case, I suspect the one which I thought sounded like a younger bird, in retrospect, it was probably just weak cry of the 'victim'.
It went on for at least 10 minutes (in fact, we left before it had finished), and at one point they all chased the victim away. However, it swiftly returned, and the others then resumed the swooping and pecking.
I know it's the law of nature but I still find myself feeling rather sad about it.