I managed to get in contact with the main person on salvo’s page via email and it seems the mystery continues. By the looks of things Salvo/BMG do not own the digital rights so they can’t really help there.
I honestly can’t think of who could own the digital rights though. The only other company I suspect that owns that is Whild John Music which is owned by Noddy Holder & Jim Lea but I can’t think anyway to contact the company apart from snail mail.
Edit: I just looked at Slades spotify page. According to the page they have over 1 million listeners!! Surely that’s more than enough for Spotify to look into getting them onto their service ( if they aren’t already trying of course).