errors creating mklink to move where iTunes backs up iPhone

My laptop (Nitro AN515-55, Windows 11 Home, OS Build 26100.6584) is rapidly running out of disk space. To help overcome this I am trying to move where iTunes backs up my iPhone to.

I am trying to link it to a directory on my NAS, \\servername name\Public\iTunesBack, where \\servername\Public has been mapped to P drive on my laptop.

I have found some articles (including on reddit) describing how to do this.

But (and you can guess there is a but) when I try to do the mklink stage I am getting errors. I have tried running

mklink /J "%AppData%\Apple Computer\MobileSync\Backup" "P:\iTunesBackup"

but I get the error message

Local volumes are required to complete the operation.

I have tried:

mklink /D "%AppData%\Apple Computer\MobileSync\Backup" "P:\iTunesBackup"

but I get the error message

Cannot create a file when that file already exists.

I have tried various combinations of using the full directory name c:\User... rather than %APPDATA% and \\servername\Public\ rather P: (which is where it is mapped to).

I have tried with " and without " but to no success.

I have tried with the directoy P:\iTunesBackup existing and not existing.

There is over 2Tb of free disk on the NAS so space is not an issue.

Any thoughts/suggestions/hints./tips/help?

Many thanks in advance

John