I would have fitted socket head bolts if I had the right size.
This is what I ended up doing, using M8 socket head bolts. Before that I took a Forstner bit to the wooden rails and cut out enough so I could secure the bolt to the rail with a nut that would then sit inside the pocket so created. Then it was a matter of lowering the seat onto the rails so the protruding bolts would go through the holes in the floor. Although this worked well for tightening, it was a bit of a faff to get the bolts to line up, hence the move to the socket heads.
