Hi Trev
I can give some advice on an S2 as this is what I did with my S2 Coupé ... but I don't know if the S1 is different in that regard
Perhaps the first thing to check is if at the base of the rear bench seat there are standard eyebolt mounting points. If you pull the bench seat out a little then on an S2 there are pre-installed mounting points. These together with the normal seatbelt mounting points make it very easy to fit, say, a four point harness.
In terms of seats I happened to go with Cobra Monacos but I guess it depends what kind you're after in terms of look/function. You could do worse then have a peek at, say,
https://www.europaspares.com/38/SEATS.html for ideas.
I found that with my seats if I used runners they sat too tall so I opted to simply have them in a fixed position without runners ... I'm the only person (allowed to!) drive my Coupé so that's not an issue for me and I don't have any two foot tall friends who need to sit in the back! I got some simple 'L' shaped brackets made up at a local fabricator; one side of the 'L' bolts directly to the seat with the other to the normal seat mounting points on the floorpan.
I've now had these in the car for over 10 years with no problems. The missus has to have a cushion though as she find the seats a little, um, unyielding!
