So I'm up and running again after a bit of a break. I've started assembling the rear tub frame and the roll bar.
I'm stuck on the seat belt mounts. Putting the book spec roll bar cross frame on and mounting the bushes on or even in results in the mounts not lining up all that well with the holes in the seats. I'd have to screw the belts underneath the cross bar so I'm thinking I may follow others who fitted a bar to the chassis itself and put the mounts onto that.