What I am wanting to do is use my main workstation for development of new Mods/Skins and some basic testing within the game, and then have my sim machine purely for driving on. However I think with the current licence that would need me to purchase 2 licences. One for each machine. Is that correct?
Nope, you can buy one licence, and install and activate the game on both machines. The only limitation is that you cannot play with both at the same time.
No, one license. Just activate it on each machine. It is tied to the user (you) more than to how many machines you have.
Does the same apply for running a dedicated server on 1 PC and playing/driving on another with the same license?
I think dedicated server does not need a license (you only need it for the launcher but you can run the dedicated server directly).