Thanks for the reply and the video link. So if my Fenix already has a high amount of fuel on board I assume I should reduce this in the MCDU before I start the refuelling process - is that correct?
Yes, and it's valid for every airplane: the new quantity requested must result in a fuel increase.
This video doesn't address the other part of my question regarding boarding the passengers. Please could you advise.
That's because you haven't given any detail whatsoever about the passengers problem. You haven't said where you parked ( Parking or Gate ).
If there's a jetway, the most common case of passengers not being visible, is the jetway hasn't connected or hasn't been detected to exists due to a scenery conflict, or has connected badly, partially or not at all. In all these situations, GSX will disable visible passengers, so you won't assume GSX has a bug because it would otherwise created floating passengers in previous versions, before the SU12 Navdata update, where we couldn't get enough information about jetways from the sim.
However, in all the cases where passengers have been disabled visually, GSX will give you a text message explaining why so, pay attention to it.