I am in the process of fitting some harnesses. They are five point and they will be fitted through the floor with a spreader plate. Does the spreader plate go underneath the car? Inside the car I am assuming that you place a washer on the bolt then the harness bracket and then another washer before pushing the bolt through the floor to screw to the spreader bolt? Hope this makes sense.
Often the lap strap is attached to a bar that runs under the seat like this. As the add says Lotus sell this part too.