I tried following tommylight's code from #210401 but got the following response:
$ sudo apt-key adv --keyserver hkp://keys.gnupg.net:80 --recv-key EF1B07FEE0EE663E
Executing: /tmp/apt-key-gpghome.BTJoQtybQP/gpg.1.sh --keyserver hkp://keys.gnupg.net:80 --recv-key EF1B07FEE0EE663E
gpg: keyserver receive failed: No name
Ignoring that and proceeding produced:
$ sudo apt-add-repository 'deb http://buildbot.linuxcnc.org/ stretch 2.8-rtpreempt'
paul@paul-OptiPlex-9010-AIO:~$ sudo apt-get update
Ign:1 http://mirror.internode.on.net/pub/linuxmint-packages tricia InRelease
Hit:2 http://mirror.internode.on.net/pub/ubuntu/ubuntu bionic InRelease        
Hit:3 http://mirror.internode.on.net/pub/ubuntu/ubuntu bionic-updates InRelease
Hit:4 http://mirror.internode.on.net/pub/ubuntu/ubuntu bionic-backports InRelease
Hit:5 https://linux.teamviewer.com/deb stable InRelease                        
Hit:6 http://mirror.internode.on.net/pub/linuxmint-packages tricia Release     
Get:7 https://repo.skype.com/deb stable InRelease [4,502 B]                    
Hit:8 http://dl.google.com/linux/chrome/deb stable InRelease                   
Hit:9 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu bionic InRelease
Hit:10 https://download.mono-project.com/repo/ubuntu stable-bionic InRelease   
Hit:12 http://security.ubuntu.com/ubuntu bionic-security InRelease             
Hit:13 http://archive.canonical.com/ubuntu bionic InRelease                    
Err:7 https://repo.skype.com/deb stable InRelease                              
  The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
Hit:14 http://ppa.launchpad.net/inkscape.dev/stable-0.92/ubuntu bionic InRelease
Get:15 http://buildbot.linuxcnc.org stretch InRelease [28.0 kB]                
Ign:16 http://dl.google.com/linux/earth/deb stable InRelease                   
Hit:17 http://ppa.launchpad.net/kritalime/ppa/ubuntu bionic InRelease          
Err:15 http://buildbot.linuxcnc.org stretch InRelease                     
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF1B07FEE0EE663E
Hit:18 http://dl.google.com/linux/earth/deb stable Release                     
Hit:20 http://ppa.launchpad.net/libreoffice/ppa/ubuntu bionic InRelease        
Hit:21 http://ppa.launchpad.net/makehuman-official/makehuman-community/ubuntu bionic InRelease
Hit:22 http://ppa.launchpad.net/obsproject/obs-studio/ubuntu bionic InRelease  
Hit:23 http://ppa.launchpad.net/stebbins/handbrake-releases/ubuntu bionic InRelease
Hit:24 http://ppa.launchpad.net/webupd8team/java/ubuntu bionic InRelease       
Reading package lists... Done 
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://repo.skype.com/deb stable InRelease: The following signatures were invalid: EXPKEYSIG 1F3045A5DF7587C3 Skype Linux Client Repository <se-um@microsoft.com>
W: GPG error: http://buildbot.linuxcnc.org stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EF1B07FEE0EE663E
E: The repository 'http://buildbot.linuxcnc.org stretch InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration

Finally simply attempting to install produced:
sudo apt-get -y install linuxcnc-uspace
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package linuxcnc-uspace

Where do I need to go from here?
Linux Mint 19.3 Cinnamon Ver 4.4.8
Kernel 5.4.0-77-generic
Cheers
Paul