Ok, got it working.
Had to uninstall Civ5 and steam, then re-install from disc, then go to steam and give it the new passcode (which it did not give me the option of doing before)
Is it a huge improvement over vanilla Civ5?
I would say yeah. 14 new cultures, and they have tightened up the bonuses for each culture, more distinctive ages, better diplomacy and the addition of spies. Far better trade options with trade routes (land and sea) that also gain you more than just gold. The Addition of Tourism to cause a culture victory rather than just a big monument to your cultural philosophies was really good as well.
The big upgrade is a really good religious system that actually impacts on not just you, but your neighbours as well.
Great game, really recommend it if you are into turn based strategy games