If you don't mind, could you keep us up to date with the outcomes,?
On the fourth trip to a dealer, the problem was eventually diagnosed as loose heat shield with stones and branches getting stuck in the open space. This has been fixed and it does seem better.
Most creaking was actually coming from the door seals and sunroof. They weren't able to fix this entirely, despite lubricating the rubbers and realigning the doors and sunroof. This was disappointing.
So, I went home and reapplied some Gummi Pflege myself, including to the sunroof rubbers and flocking. This finally seems to have fixed the "old ship" creaking. It must have been a combination of the door and sunroof realignment
plus the Gummi Pflege. I know this because I had tried the Gummi Pflege on its own initially and it didn't fix it.
Hope it doesn't come back, but at least I've isolated the cause and fix. Such a relief to get in a new car and be able to drive it without the annoyance of loud creaking throughout the cabin! I know it's not German saloon, but I still expect a new car to sound and feel like a new car.
Levels of service and responsiveness from Holden dealers and Holden Customer Care was just abominable. They were blaming each other and giving contradictory advice. Hopefully I don't have to go back for anything else!