Your Greenby migration does not require any changes to your existing Plusnet FTP settings. You should only require the below information if you're setting up a new device, or have lost the original settings.

Plusnet
ftp.plus.net or homepages.plus.com

Force9
ftp.force9.net or homepages.force9.net

Free-Online
ftp.free-online.net or homepages.free-online.net

FTP Port 21