Hello m8, whilst restoring my S2 I fully stripped the interior including all of the seats......the rear seat was stripped, the material put through the washing machine and the steel backs were cleaned up, rust proofed, primed and sprayed in Satin Black before reassembling.
Be carefull if you strip the sestscas the material is held in place by some very sharp clips that are very delicate and prone to snapping! The elastic is held in place at the rear of the sest by a plastic clip that is pushed into the seat back, the elastic then goes through the seat and should protrude through the plastic oval shaped insert in the front of the seat - this plastic insert is in 2 parts that sandwich the seat material inbetween, I removed these before washing the material. I gave the old elastic material to my Mrs who found some new elastic in a shop and bought about 1 meter of the stuff then cut to length, stitched over the end that is held by the clip in the resr of the seat and at the other end stitched over the end then made a small loop to hold the small plastic bar that clips into the seat belt clasp.......this is shaped like a small barbell, I only had 1 on my car so I took some measurements and then made some new ones using a lathe.....I made these out of steel instead of the stock plastic. So to refit I put the elastic in the clips, refitted the clips in the seat backs, replaced the foam remembering to thread the elastic through the holes in the foam. Refitted the plastic mouldings to the seat material, refitted the steel tie bars to the foam and reattached the seat material by the small steel rings that hold onto the bars before refitting the seat materual to the seat back. I also put a large safety pin thru each piece of elastic to stop it being taken back into the seat, the barbell was inserted into the looped end of the elastic before finally clipping the seat belt clasp to this......reapeat for all 3 and they should look fine and stay upright against the seat back