An update on the Macbook Retina creaking issue. I took it to the Apple store and they wouldn't replace it, but tightened it up for me and told me to come back after some use, to see if it had improved. Well...it hadn't. It lessened slightly, but the issue persisted.
Rather than go back to argue for a replacement, I decided to get a Retina screwdriver from eBay myself. A nice high quality magnetic, aluminium handled one. Well, I took off the rear plate, and then re-applied each screw. But one by one in opposites. I,e, far left followed by far right, top followed by bottom etc. To ensure pressure was applied evenly throughout.
I only tightened each to when I just started feeling some resistance, instead of going super tight, and low and behold, no more creaking. There's some rubber sealant around the rim below the Mac plate under each screw, so I guess some flex is fine.
In any case, too much torque and tightness is the problem. Loosen the screws slightly (one's that are too tight) and it will solve the issue. Fingers crossed the creak doesn't come back!