Well as there is no America's Cup racing on tonight, thought I would repair my car seat
Removed it from the Mog - that was a hard job - bolt heads spinning where it was inaccessible to get a spanner in but overcame it by jamming a metal file in the chair runner - you can see it's yellow handle in the photo

The reason for the saggy bottom is because the rubber mesh that forms the bottom of the foam (is it called webbing or a diaphragm ?) has split neatly in a straight line letting the foam seat padding come out

My idea for a fix is to put metal eyelets in each side of the spilt rubber and lace up like a shoe
Here goes (TM silence for a couple of hours)