I never got back to you on this - which was rather rude of me, sorry - but yes, this was a Web Application Firewall thing in the end, over which I had no visibility.
Hi Martin,
How did you end up fixing the issue?
Kind Regards,
Charles
Hi Charlie, the Web Application Firewall had its own file size upload restrictions on it that were more strict than the Moodle/Apache/PHP settings, which it was imposing. I had no idea the WAF was doing this, so hadn't thought to ask, but that's where the issue was and the firewall guys fixed that for us (or, we could say, "un-broke" it).