So it's like this ...
I used to have one account for all the family but the playlists were getting clogged up with my sons crazy tunes. So I upgraded from premium to family with one extra account and gave that to the rest of the family for use on the shared computer. Great.
I needed to transfer 20 or 30 playlists from my personal account onto the family account and the best way I could find to do this was to make my personal playlists public, log off the windows client, log on as the family account and copy the playlists to playlists in the family account. Great.
When i'd finished, I made sure the playlists on the new account were private. Great.
Then I logged off the family account and logged back on as my personal account, made the playlists secret then deleted them. Great.
Now to check that everything was cleared up, I logged back on to the family account and checked that my personal public playlists had gone from my personal profile but they were still there. They were not even listed as playlists on my personal account and profile but were still visible and playable from my personal profile as seen on the family account. The only way I could remove them was to uninstall the windows Spotify client and reinstall it.
I tried deleting the 'storage' folder that Spotify uses to store temp files but this didn't help either. I don't know whether this is a bug and it's not really that problematic but I thought i'd report it here.