Now I was able to remove the four 13mm hex head securing bolts. The rear one in the tunnel was a pig. It cracked and screamed but a long tube on the wrench got it out without snapping it. The bolts are all held with "Loctite" that makes them really stiff to remove.
When I finally got the seat out of the car, I realised that the seat belt receptacle was attached to the "shock absorber" thing via a bowden cable!!
I read the manual and discovered that the dangerous shock absorber was actually an exploding piston assembly. In the event of a collision, it explodes outwards and tightens the cable, pulling the seat belt in by a few centimetres. Guess what a new one was going to cost? No, I didn't even bother to ask.