Having now resolved my issue, I thought I'd provide an update in case any use to others.
On the sport seats at least, the material part of the seat is attached to the metal seat mounting frame by four lifter units which are 90 degree gearboxes that take the drive from the motors at the front of the seat and turn this into up and down motion to lift or lower the seat. In my case, one of these had developed excessive play, creating a most disconcerting backside instability. To make this worse, the sponge on the drivers seat was a bit deteriorated in the swab, making it sag a little too much and lack support. I'm suspecting a gentleman of a larger frame may have previously owned my car...
So, my resolution... With the seat removed, the offending lifter was the rearmost outer unit which had developed a little wobble. To get access to the lifter, there's just a few bolts holding on the side of the frame to remove and a nut holding a plastic sliding runner. Once removed, the metal plates on the bottom of the lifters tend to easily fall off. This isn't such a disaster as the parts are easily refitted but it's a job where you could do with two sets of hands and everything seems to constantly fall off. Getting to the top 19 nut on top of the lifter is a pain at the rear because it's up inside the seat. A ratchet ring spanner with a swivel head is the best solution. In my failed lifter unit, it seemed that the bearing race was worn at the bottom and potentially the plastic seal at the top. I replaced both and this removed the play. Refitting was straight forward, applying some locking fluid as necessary to fittings. When tightening the top lifter nut, bear in mind it has a spring fitting to allow some movement even when tightened.
Failed lifter
https://pin.it/AtlLnoZ
Lifter removed
https://pin.it/450CNP1
So that was the wobble resolved, now for the sagging swab. I couldn't see any issue with the swab springs or metal clips, so I was pretty sure it was just deteriorated sponge. My understanding is that at the moment, replacement swab sponge is unavailable, so until I fancy a full seat rebuild from somewhere like Southbound, I wanted a DIY solution. Enter, some very expensive sponge made in the USA specifically for the task of supporting sagging furniture. This product, "Sagsaway" is a bit pricey for what it is, but it is the ideal material... Very light, but a robust, dense type of sponge that is designed to be inserted between existing sponge and the springs. This was fairly easy to achieve by unhooking the swab material flap at the rear and gradually working one strip up the middle part of the swab (see pics). Refitting the seat is the reverse of removal.
The result is far better than I could have hoped and a substantial improvement in swab support just from the one strip. Visually of course it's invisible once fitted.
Rear seat flap removed, working strip behind springs
https://pin.it/15I0Mfy
Seat reassembled with support strip in place
https://pin.it/4p078Uh
No more seat wobble and a considerably less saggy bottom...
