@Magnet @Coogs
UPDATE:
So, we ended up doing a fuse pulling test the other day to try and find the offending circuit, and ended up figuring out what was draining the battery! Here's what we did:
1. We linked the multi-metre (set to 10amp mode) to the battery to try and detect the drain. We noticed the standard cooldown drain once the car has been locked, which settles after a few minutes to around 300mA - which is obviously still high!
2. After finding the 300mA drain, we began fuse pulling taking out the fuses one by one in the engine bay fuse compartment, and eventually found that fuse F20 (Navigation and Phone), once pulled, reduced the drain down to about 10mA, which is far better! We did some research and found that it was linking to the old telephone mount in the car, which a feature available to Europe at the time since phone usage while driving was still legal in some countries, namely Germany, but was removed when imported to Australia by law. So we guessed by disconnecting the cable (for which was found by removing the rear ash tray behind the centre console), that it would remove the drain - however, this didn't actually fix the issue for whatever reason.
3. After disconnecting the old phone mount cable, we actually just decided to remove the fuse as a whole, disconnecting that circuit and trying to figure out what might not work (we figured the RNS-E's navigation might be impaired once this fuse was removed after looking at the wiring diagram), but interestingly, everything still functioned perfectly fine! So we've just settled with keeping that fuse removed for now, and the drain has stopped without any noted issues stemming from it!
Hopefully this solves some other people's problems too!
Thanks for the help guys,
Jasper