Ok, so I managed to track down the guys who fabricated the modified rear bumper that crops up on here from time to time.
www.a-tm-engineering.co.uk
They say they did it around 10 years ago.
Anyone interested in this mod, or a front bumper for that matter, can now get hold of them via the website.
They need the vehicle so they can replicate it in house. However, I suspect once they have done one, they can keep the dimensions if others are interested in having the same mods done rather than everyone having to leave their vehicle with them.