I decided that I was going to give it a go from the inside and after a little bit of surgery.......success
If anyone has this issue then these are the steps I took:
Firstly remove the the bulb and ignitor and holder which should leave you with the below. Then proceed to remove the four T25 screws highlighted (don't drop them)
Remove the wiring connector that is attached to the small circuit board. Then remove the cage by pulling towards you and carefully twisting to avoid the other components (its one piece). It will look like this.
Now that the lens and cage is out you should be able to put your whole hand into the light and pull the ring back on. Note there is a right and a wrong way (the thick bit is at the bottom - compare against your other light). Careful of the little tabs. I wore gloves to make sure no fingerprints appeared on the lens from the inside.
After resolving the issue I plastic welded one of the clips to stop it coming loose. Be careful to make this neat though, as you will see it from outside.
Reassemble and the job is done.
Now to find the rest of the bits for my retro fit
Thanks to all for the advice.