Tom,
To gain read/write acess to the etc directory via CIFS, make sure the account you are using to access the filer is in the filer's local Administrators group, and that "options wafl.nt_admin_priv_map_to_root on". It also might help to put the NT account you are using as mapping to root in /etc/usermap.cfg.
Thanks for the suggestions - neither of those worked, but I realized that I'd done a useradmin useradd xxx where xxx is the NT account I was coming in as - these appeared to clash, i.e. if xxx was defined as an administrative user on the filer, I couldn't do remote admin as xxx. I simply removed the accounts on the filer and everything is great now...
Thanks!
Ross