Flyerncle saves the day again!
Just an update on the roll bar situation. I finished fettling down the ends so that it was entirely level and the outer edge of the each side of the roll bar was 26.5mm from each edge, which to my eyes looks bang on in the middle of the plates.
I've also cut out a 50mm hole in the middle of one of the plates for the roll bar to sit though, as suggested by Flyerncle and HandyAndy, to give the joint some extra strength. I'll be doing the other side tomorrow and then welding it up, to quite a while to do with my pillar drill, hacksaw and rounded file, if only i had a 50mm slightly oval hole saw
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Cheers for the help everyone.