Hello PRP_53
Thanks, and sorry for delay in reacting.
I tried the Terminal route for one of my admin accounts, but this did not work because the password it requested was not recognised.
I had been concerned that i could not recall the p/w for the second account but then tried anyway. So, signed out of one account and then in with the other, and was then able to upgrade to Sonoma.
Signed out and signed back in with my preferred account. All ok.
So, it seems - on my setup at least - that even if there are more than one accounts with admin privileges, only one actually allows OS upgrades and only if signed in with that account.
Odd about Terminal, because I use the same p/w for both admin accounts, so should have worked (?) whichever of the admin accounts i tried first.
Thanks very much for jumping in so quickly with your help.