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I used the Plesk control panel to add a LetsEncrypt certificate
Plesk "SSL/TLS Certificate... Secured components" shows "domain photocomp.uk" and "*.photocomp.uk"
When I attempt to use Filezilla to access the domain vai FTP I get "The server's certificate is unknown"
Under "Subject of certificate" the error message shows "secure16,host-it.co.uk" and under "Session details" it shows "photocomp.uk:21 - host name does not match certificate."
I've tried both of these as the "host" for Filezilla but it still fails
Yes, that worked and HTTPS connections to your IIS server use that new cert. HTTPS works by default on port 443.
You now have to configure your Filezilla FTP server to also use that cert. I don't know Filezilla very well. But, I found these docs at the Digicert site and these look like good place to start. Just ignore the part about the CSR since you already got your cert. Use the names of the cert files you got above instead of the ones in their example. In step 5 it would be the file named fullchain.pem (I don't know what folder your system puts them)