Ok so I went to see Jana Lorso but there were no dialogue options to get the credentials - I have already returned the Ithorian Droid to her and slaughtered a party of mercenaries in her office - maybe I've left it too late?
Anyway, I have a bigger problem. I've just killed the Exchange boss Slusk and Luxa but now I don't know how to get my ship back and the mission information in the Journal is still telling me to speak to the exchange regarding my ship and then Luxa has set up a meeting with Slusk for me? Doesn't look like I can progress any further - am I missing something or do I need to load from a previous save?
Hm, that's strange. If you've already returned the droid to Lorso, you must have had the quest to do so, and that comes with the fake credentials, otherwise you wouldn't have been able to enter the hangar in the first place, to get the droid.
It could be that you can't use the credentials now, since the droid has been stolen and the quest is no longer active, in which case you're stuck there. You could try persuading the Ithorian, by equipping all Persuasion-bonus-giving items you have, but it may auto-fail regardless, I'm not sure if it's possible. In that case your only option would be to reveal the smuggling business to Lt. Glenn, chief of TSF, and then betray him, if you want to stick to the dark side path.
As for your general progression, Lorso should have given you the quest to kill Slusk after getting the droid. The mercenaries in the Czerka offices only appear once the Exchange is destroyed on Telos, so you should be more or less done on the station. Go to Lorso and talk to her about the mercenaries, a reward and your ship, she should send you to the docks then. I don't want to spoil you, if you haven't played the game before, so I won't go into further detail.
May I ask, do you play with TSLRCM installed? That's the "The Sith Lords Restored Content Mod", a community patch which restores a lot of content that didn't make it into the game because it was rushed out back then; and also fixes a ton of bugs. It makes the game much, much better and is generally recommended for everyone, first-time players too.