It would be reassuring if I knew why they stopped working but I’m stumped. The map tiles (the square graphics that make up the map’s background graphics of streets, rivers, etc.) are provided by Mapbox which has a free level of access. But these stopped loading, with errors saying “403 Forbidden”. There was nothing in my account at Mapbox to suggest anything was wrong, and we were well within the generous limits they allow.
In the end I tried changing the access token – a long string of characters that links the maps on this site to my account on Mapbox – and that seems to have fixed things. I have no idea why, or what the issue was, but I guess that’s all fine.