In the past I have downloaded three files, a certificate and the associated files, and upload them to my home-based web server. All good.
Now I cannot find how to do this. Am I going mad, or has the ability to just download the files been removed?
I am using an QNAP NAS running their hybrid Linux version, QTS. It is locked down so I cannot use CertBot. Plus their own integration with Let's Encrypt does not seem to function.
Let's Encrypt only offers an API (with the ACME protocol) to be used by software, it has never offered a website where you can "just download files" (i.e., the certificate/private key). You might have used a third party webbased ACME client in the past. Such websites are not run by Let's Encrypt.
If you want a simple software to easily get a Let's Encrypt certificate (and the files you seek), you can try using my ACME client CertSage. Does your NAS support PHP and do you have port 80 open from the internet?