As Jocko suggested I manually updated the current version 8.0 with the 8 files changes and 1 new file and the Spotify plugin now works as expected!
I've attached the files just in case someone else encounters the same issue and is comfortable manually updating the files. The files already have the correct owner/permissions which may be retained if copied using Linux/Mac systems.
I updated my backup NAS first, and that now works perfectly. I then updated my main NAS and whilst the Spotify plugin is working fine it does not launch automatically ( I have to manually start it). I believe this is another issue which I may have introduced whilst playing around trying to get the Spotify plugin to work.
As for version 8.1 - thanks for preparing this so quickly fvdw!!! Seeing as my main NAS is not running perfectly I will try a fresh install of 8.1 and report back shortly.