I don’t know if Apple, and Google upped the ante, or I had not noticed before, or the SSL tools on the server had not kept updated, but I started getting warnings that my site was not secure. Letsencrypt has done a pretty good job for the last 5 years so I was confused as to why suddenly now , the browsers no longer liked up. Even harder now that MS Edge do not let you easily view the full cert.
A quick test at SSLlabs confirmed that the SSL for farcorfe.org.uk was all ok, but the sub-domain of www.farcorfe.org,uk was the actual issue. The SSL cert had not been validated for the www redirect.
Some quick Googling found this page – https://stackoverflow.com/questions/41097696/letsencrypt-certificate-for-www-and-non-www-domain with the required Certbot commands to add the www to the cert and a restart of Apache got the issue fixed.