Ok, Here's the lowdown.
The front and rear nuts of the rear mounts are already in the roof chassis.
There is a small plate in the edge of the roof lining that is spot welded in two places (two spots) to the vertical edge of the roof skin - NOT the roof chassis. This plate has the thread in it which the aftermarket bars screw to.
The factory option if you ordered roof rails was to replace this little plate with a larger one that had the after market screw option but also had two M6 nuts welded to it. It's is also spot welded in a further three places to the horizontal roof skin.
In order to not have to unpick any welding and to avoid all the expense of painting I cut the downward bent edge of the plate slightly.
This allows the plate to slide in but the edge of the plate rests on top of the smaller plate which is already in there still retaining some of the strength of the vertical.
I used the old skin of a scraped sportswagon from which I got the bars to mark out the front holes. I then used the bars themselves to mark out the rear holes.
I first put cellotape over all marks to prevent and drill skidage or paint flaking.
I then centre punched the mark for the front holes and drilled a pilot hole (2mm) for the rear most hole of the FRONT mount first as if you look up from inside the roof chassis you can see a space that accommodates for the nut on the plate that is put in at the factory. This way I could tell that I was in the right ballpark. I then drilled a pilot hole for the front most hole of the front mount and held the plate to it to measure it from the rear hole. Looked good so I drilled them both out to 8mm.
I used Liquid Metal (available in a silicone style tube for a silicone gun thing) and covered the plate in this, shoved it from the inside between the roof lining and roof chassis and then aligned the bolt holes.
As this stuff only allows you to move it for 10 mins before it sets, I quickly lined up the rear holes, centrepunched, and drilled these at 8mm straight off. First bar was about 1-2 mm out so I had to carefully grind a but more away with the drill bit to get the bolts in, 2 rear mount was so spot on I laughed.
I then screwed the bars down tight. The bolts had the bolt head, a small washer, then a large washer, then (and you can't see this in the pic as it's under the large washer) and 13mm diameter, 2mm thick rubber washer.
Once mounted, I filled the gap between the front plate and the roof chasis (not the front plate and the roof skin as this is now bolted tight together) with more Liquid Metal to bond them together.
All in all, I'm happy with the result. The bars are solid as any other bars I've come across. I'll come back and post pictures if I hit 110mph with my roof box on and suddenly find I'm driving a convertable