smbclient NT_STATUS_ACCESS_DENIED

For users who are using for the command line option, use

$ sudo smbpasswd -a <user_name>

this will prompt you to assign the password.

WARNING: This refers to Samba 2. We are at Samba 4 now. Take care which version of Samba you are using. As stated in my comment, the GUI will break your configurations.

A work colleague has pointed me in the right direction:

The Linux user ID being used to access the Linux share needs to have a second "samba" password defined for it. The easiest way to do this is to install and run the GUI Samba Server Configuration app, which isn't installed by default.

The Samba documentation does explain this, but it's buried in the masses of documentation explaining all the various arcane aspects of samba.conf configuration etc.

The following article gets to the heart of the subject:

http://ubuntuhandbook.org/index.php/2014/05/ubuntu1404-file-sharing-samba/