Have you seen the bits of plywood that come with the series 3? in my car ( still building) i have reinforced the coach bolts with large plates inside and square tube outside then bolted through the floor in to a subframe that is bolted to the gearbox support at the front and a rear cross beam. It was to this substantial construction i was going to fit the chairs. The back of the RHE supplied seats is just held on to the base with some plastic hinges and rests on the seat support cross bar, with the base rigidly fixed the seat backs fold a little forward to slot hood or side screens behind, it is the seat belt that secures you in the car. Now this is where your experiences come in, I have looked through the SVA regs and it says nothing about the seat construction but does show a recomended seat attachment rail (looks like the stuff aircraft seats are attached to) at this stage i just intend to use the stuff supplied by RHE more expensive seats will come later on if this method fails. Has anyone had SVA troubles to do with their seats?