After another user installed Spotify from the Software Centre, I noticed that I could no longer apt update as so:
Err:2 http://repository.spotify.com stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4773BD5E130D1D45
W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4773BD5E130D1D45
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
So I tried the man page suggestion:
sudo apt update --allow-unauthenticated
But that did nothing and nor did the apt-get version, so now I'm in a bit of a tricky situation. Any help would be appreciated, thanks!