Eventually found out the cause of the problem - me!
I had set up Mount Manager correctly on the Main Menu>Setup>Network>Mounts>Browse network neighbourhood>MOUNT MANAGER.
However, on the pre-screen, the Browse Network neighbourhood: "Main Menu>Setup>Network>Mounts>Browse network neighbourhood", I had left the screen blank rather than keying in the IP address etc., of the remote box using the Expert option (blue).
When I set this up, everything worked fine.
Thanks for your suggestions (and your patience!), guys.
David