Thanks for the replies, drilled the rivets out and used a nut and bolt. After spending ages bolting the seat back in it was still shifting. Turns out that the joint between frame and seat was connected in the same fashion. So out it came again and managed to drill out the rest of the rivets and put in bolts.
Cant see how these were every designed to last, a big square whole with a large flange headed rivet to take all that wear as the seat moves for adjustment.
I expect fixings on modern seats have moved on a bit.