AHA!
Samba was the clue. Uninstalling and reinstalling Samba, I filtered the available apps by the word Samba, and there was WinBind installed. I then recalled that I had installed it a while ago, thinking I've heard that name before; I probably need that. I have now removed it and seem to have fast logins again.
Please feel free to point at me and call me a numpty.
Cheers Steve