Apologies folks!
As Andy says, I migrated the BAA website to a new server last night, and that is almost certainly the cause of these teething issues. I’ve actually seen exactly the same issue that Jeremy and Chris describe, but I wasn’t able to reproduce it, as once I’d hit refresh once, it seemed to completely disappear.
On the old server, we used a tool called varnish, which encouraged web browsers to cache pages when users weren’t logged in, in order to reduce the load on the server. The caching should have been inactivated whenever you logged in.
The new server is a lot more powerful, and varnish is a bit of a troublesome beast, so I’ve tried experimentally turning it off for now. I’m surprised turning it off would have caused the issues people seem to be seeing, but it seems like the most likely cause.
If the culprit is indeed old cached data from varnish, my guess is that this issue will probably entirely disappear within 24 hours as the caches in everybody’s web browsers expire. But please do let me know if anybody has longer term issues.