Found the source of the problem. I tried the address from a browser and got an in-your-face warning from BT that the site is suspicious. I recalled that recently I had an email from BT about their web protect service, but I thought nothing of it. Turns out it has been activated and the BT web protect service is tagging the OSMC server as high risk. I disabled the BT web protect and downloads are now working again.
I doubt I’m the only OSMC user who has BT internet, so unless anything can be done server-side to prevent this false-positive, it might be worth having this issue posted prominently?