Are you referring to the black bolts which hold the seat assembly to the car body?
You cannot go replacing these with just any bolt, especially with stainless steel ones, which aren't even as strong as standard tensile bolts. They are high-tensile bolts which take a serious load in the event of an accident and stop you flying out with the windscreen whilst still sitting in your seat.
Please don't get replacing safety critical parts with just any old bolt.
Edit: Also, the original bolts are Torx head, not Ribe, as is written all over the instructions from the MG-Rover workshop manual. Probably why the heads are damaged!