Finally bit the bullet and upgraded by 32bit iMac to latest OS. Resentful that had to pay to update Office, but other 32bit software was already capable, or I can live without, so time to do so.
All went smoothly except for passwords.
Firstly, as part of the setup it asked for the Admin password. Huh? Damn, havent used that in 10-years, no idea, so off to the Terminal to reset it.
Secondly, now every time I start up its wants me to login, despite automatic login for me being set in Users. Im the only person who uses this iMac, anyone know why auto login isnt working? Have turned it off, and on again, bit still wants me to login.
Second question re Big Sur is, is it still possible to backup iPhone and iPads to the Mac? Looks like that is no longer an option in itunes.