Not sure if this is the correct language but I will refer to Main User (the initial one set up for the Virtual Server) and Additional User (others setup after this).
When an Aditional User is created in Virtualmin the group not set the same as the username so umask is set to 0022, not 002. The group is set to the same as the Main User for the Virtual Server. This means User Private Groups user access does not work (the Additional users are put in the group for the Main User on the virtual server)
So, for Virtual Server tvppclientarea.uk the Main User is tvppclientarea with a group of tvppclientarea. An Additional User, admin@tvppclientarea.uk is set up with a group of tvppclientarea, not admin@tvppclientarea.uk.
This means that when admin@tvppclientarea.uk uses SFTP the permissions are set up wrong (group permissions are not set up as write).
So if another Additional User' for the same virtual server, i.e. bob@tvppclientarea.uk, tries to overnight the file it fails.
Side note:
the umask is set in /etc/profile, the relevant bit is:
UID -gt 199 ] && [ "/usr/bin/id -gn" = "/usr/bin/id -un" ]; then
umask 002
else
umask 022
fi