I found a piece of metal and a spring on the floor, and have discovered that the part comes from the inner rail (the seat rail nearest the arm rest) - it is a stopper which sets where the seat should sit in its normal position, which explains why it will slide all the way to the back when folded forward.
Having compared to the driver's seat, I think I am missing a small pin to hold the metal plate in place, and I think also whatever the spring fits on to. Hopefully this makes sense. Has anyone else had the same issue and found a replacement part kit? Keen to try to sort this myself rather than a dealer trip if possible, as it was only recently serviced and MOT'd, so I'm not intending to visit the dealer again until I head in to order the new car.

Thanks