Oooof, Nottinghamshire - where abouts?
Basically, the rubber is p155 poor in the mounts and will ultimately sagg so to help support the rubber mount that is in the shape of an X, you can support the bottom part of the X.
Basically, imagine a square, an X in the middle, and a circle in the middle.
The exhaust slides in through the circle, which is in the middle of the X. The bottom half of the X saggs under the weight and movement of the exhaust, so in order to support the X and keep the exhaust hidden under the bumper, you need to wedge something there, or replace the rubber to something more durable. I'm sure Fred could provide a link to somewhere?