Looks like good news: Most of the music is now back on Spotify. But there still seems to be problems with the app - when I scroll through my folk playlist, there is still the occasional track greyed out and unplayable, but if I click through to the full album the track is there. If I click play on the offending track in the full album it plays, then if I go back to the playlist it's playable there too. So I think there are probably inadequacies in the Spotify app design that mean tracks in playlists take their time to show as available again when they come back online.

What's weird is that there are a couple of albums where a select few tracks are still completely unavailable to me - like the Pentangle album "The Time Has Come 1967-1973" there are still 10 tracks which are unavailable. I suspect this might have something to do with the fact that it's a compilation album and so the licensing is more complicated?

I suspect as I said before that this whole thing was probably a licensing snafu with a couple of labels not meeting a deadline or something.