Seems the error occurs because the install program is running under elevated privileges, whereas my m drive is mapped for my standard user account. Simple fix is to map an equivalent m drive for the local admin account and, hey presto, it all works fine. Steps to follow are below (taken from the vista64 forum):

1. Disconnect the previously mapped network drive from Windows
Explorer.
2. Right click on Command Prompt in the Start Menu and selected Run
As Administrator.
3. Type “Net Use m: \\servername\sharename”
4. Exit out of the command prompt
5. Open Windows Explorer
6. Click ‘Map network drive’ menu item
7. Select the m: drive and typed \\servername\sharename into the
Folder box.

Of course if you already have an m drive mapped, then you only need to do steps 2-4.