Thanks guys, i found the correct code on the purchase site. Just to clarify things for anyone else, there are 3 possible codes but only one is correct:

20-digit Early Access code.
18-digit Activation code.
20-digit registration code found on the site you purchased the game from, which for me was never sent in an email (www.greenmangaming.com) or in the case if you have a physical copy. This is the code you want to enter in the client.