Cannot start family sharing - says account is already in family

Hi - I'm trying to start family sharing on a new MacBook but I can't get past the start screen. I worked out that I had an old Apple ID with family sharing for which I was the organiser, and my current Apple ID was a member of that family. Apple has now deleted the old Apple ID at my request, but when I try and set up family sharing with my current ID it says I am still a member of the old family, which doesn't exist anymore (I checked, and tried to login - it says 'you do not have an Apple ID'). Can anyone help please? It seems I am a member of the old family but the old Apple ID no longer exists! Thanks in advance :-)