Some Apple devices are showing a security warning with our sites.
If you click on the warning to get more details it shows that the site certificate is valid but that a root or intermediate certificate has expired:
This appears to be a problem with Apple devices not checking intermediate certificates which have expired. A very large number of websites, ours included, use security certificates from Let’s Encrypt, and their intermediate certificate was replaced on 1st October 2021.
The first suggestion is to try restarting the Mac, and if that doesn’t work follow these instructions for downloading and installing the new certificate manually.