Phew – 4.5 hours later, the problem is solved!  Let me save you 4 hours and 25 minutes, if you are submitting / getting the response below when trying to connect to your GIT repository from within a company firewall (on Windows):

$ ssh -T [email protected]
ssh: connect to host bitbucket.org port 22: Bad file number

The problem I had related to proxy authentication.  I was able to make this work by updating the config file in the ~/.ssh folder.  This is normally located in C:users[username].ssh.  If the file does not exist, create it – and add the information below:

ProxyCommand connect.exe -H [ProxyUser]@[ProxyServer]:[ProxyPort] %h %p
Host bitbucket.org
 IdentityFile ~/.ssh/id_rsa

Note the space before IdentityFile (apparently this is important).  The %h and %p values are replaced with target hostname and port when ProxyCommand is invoked by your ssh command.   But you do need to provide your system values for the [ProxyXXX] settings.   So how do you get the proxy server details (when the browser is set to auto-detect proxy settings)?  Well, the [ProxyUser] is most likely your login name (it was for me).   The rest can be worked out from this thread

Here is the command I used to pull my repository (note the lack of a slash before OWNER):

git clone [email protected]:OWNER/REPOSITORY.git

Happy coding!