Thanks Matt.
I wonder what caused the issues yesterday then - reading this it appears that rate limits are applied per user or per IP and so it was odd that the site was inaccessible to everyone (I also tried using various IPs).
The forum has been getting busier lately - we’re now serving around 600K pages a month, but I wouldn’t have thought that would trigger this in itself.
Would the rate limiter only show a 429 to the IPs affected, or everyone?
Any other ideas what it might have been or how to troubleshoot this?