Seat belt mount heght

[snapper]

.so you cant take the strap over the roll bar and secure it to a point lower down the chassis, it has to be on the rolll bar?

I think you'll find you can, the roll bar becomes the "effective seat belt mount" for the mounting point height measurement.
The real mounting points should of course be upto standard and the bar the belts loop over will be assesed for strength "as if they were mounted there" as in the event off a crash all the load would be consentrated on the mounting and the bar.
I did this at SVA, any retractable belt system uses this method as the mechanism is usually lower than the place the belt retracts into.
IVA has not changed the requirements from the SVA from what I have read.