Am I to assume that is only works from within the network as I get an error when I try to do this from an external address?

It should work OK from an external address. Can you tell me the address so that I can try it? I don't actually need a username / password to test it.

I can see the application, just when I run through the process it picks up  the external address, but says the internal one (when I tell it they are different) is invalid. 

What is the internal address you're using, and what is the exact text of the error message?

When I run it internally it picks up the server name (keserver) and I have to specify the external address. Running externally it picks up the external IP and I specified the internal as keserver then I get the error

 

"keserver" is not a valid address for MirrorSync Internal Address

 

I don't get this running internally using the same details. 

Huh, I'm not sure why 'keserver' would work differently depending on where you're running the MirrorSync admin utility, because the actual name resolution should be happening on the MirrorSync server, not the admin client. At any rate, specifying an actual internal IP address as opposed to 'keserver' should solve the problem.