Found independently ... I did - 3 of them(!)

Reboot wasn't working - I'd was the first thing I tried (a couple of times).
Oddly, whilst the processes had yum running (sort of), they hadn't locked the repositories as a manual run worked and updated and number of items. It was the fact that it ran manually whilst still reporting busy was what had me head-scratching. Very strange. It was the only one of three server exhibiting the problem.