I have pretty much exactly the same setup as Ian, minus the oem belts. I also have and additional roll bar set into the base of the rear partition. Works perfect, follows the roll hoops (roady) and provides sufficient roll over protection, and isn't in the way like some half cages I've seen (if your a tall guy with restricted movement for aft setting of your seat.)
Rubbish pic, I know, but shows what I mean.