It seems this is a common 5G trait. Over time the rear bumper sinks lower creating a gap between the top of the bumper and the rear wings. While at JapFest I had a look at other peoples 5G Ludes and saw it was a common problem. Here is what mine looked like:
To sort it, you will need to remove the rear bumper and the rear bumper support bar which is fastened by four of these rusty things:
Apologies for the bad photo (rushed taking photos with my camera phone). You will need to elongate the bottom of the screw holes of the bumper support bar with a file. Maybe by 3 mm, depending on how far it has sunk. The photo below shows insulating tape indicating where the bar was originally located. I've moved it up a few mm.
Here's the bar refitted in its slightly higher position. I gave it a coat of black Hammerite paint to help protect it from rust on top and under. While faffing around under the car, I decided to give the rear facing section of the exhaust back box a fresh coat of Halfords High Temperature Engine Enamel Paint. It last had a coat about 2 years ago and was flaking off so needed a fresh coat.
Slightly off-topic, I also gave the metal shield thingy for the 4WS actuator a coating of black Hammerite paint too:
And here's the finished product (the following day after washing & waxing the car):
In retrospect maybe I could have raised it on the right by another mm, but I'm happy with it.
there are two of them. 
