Kevin4fm said:
... maybe it was a UK thing ... I've not yet tested if it really was...
Thought I should follow up on this, in case anyone stumbles upon it in search of possible answers to their own problems!
I'm now back to connecting my torrents, using qBittorrent, through a UK VPN server, and I can access this site from the UK, both directly, and when going through my VPN. So this points to it being a "me" issue, not something wider.
One thing maybe worth mentioning. When I first reconnected through a server based in another country (Finland), qBittorrent ignored all my previous .fastresume files and rechecked every single torrent to create new ones, which now function exactly as they should. It's another variable to consider if you're having to troubleshoot.