There is a plastic carpet runner / holder that clips into the top of the sill. This runner clips into oblong slots. I'll be glugging some waxoil through them when I'm done.

Pic below (just taken) shows them.

Also shows some rust around one of the clips which looks like where the water has got in to do the rot below. Dodgy door seal rubber, inner door plastic sheet or maybe even water migrated from the pollen filter area (which is known area for water ingress). I'll further investigate once the welding is done.