You’re right – it was the cacheing causing the issue. Disabling the cacheing plugin has fixed the issue although obviously I’m now without any cacheing functionality on the site! Not your issue though.
I appreciate you taking the time to look at this. Donation on its way. ??
Thanks for the speedy reply. I’ve tried it with the default theme but the same problem applies – sorting isn’t enabled for visitors who aren’t logged in, but if you log in you can see the sorting.