As promised on another thread and requested by bg1
My drivers seat has been sagging for a while, the leather hasn't split and there wasn't too much disintegration of the foam. I took the car to an upholsterer who wasn't able to help but did confirm that the foam was not damaged and did not need replacing. These M Sport seats are bonded to the foam so the usual padding techniques would not work. Following the advice of one of the forum members I decided to take the seat out and attempt a repair using cable ties.
Check this thread which has a step by step guide (but sadly the photos are gone) and this video. Pelican Parts have a step by step guide with photos too.
There are a few videos on how to remove the seat, the main piece of advice is to make sure that the battery is disconnected after the bolts are removed to avoid airbag warning lights being triggered. If like me you prefer to let others fiddle with the oily bits make sure the bolts are really loose before disconnecting the battery as they will not move afterwards cos they are powered by electricity (!). If you are not able to get to the bolts and the battery has been disconnected and you don't have the frame of a six year old girl to get through the gap between the seats into the boot it is really useful to remember that the coupes have a manual key lock in the boot opener roundel. After unplugging the electrical socket, it is easier to take the seat out with seatbelt bolt still attached and remove that outside of the car. I also recommend covering the sills as the protruding bolts will scratch the sill protector and drivers side covers are like hens teeth to get second hand and impossible to get new (E89 M3 ones are availble new but are £65 each.
I was relieved to see that the foam was in good condition and it appeared that my backside was being supported by a wiring loom. There were holes in the frame at the front so I drilled three holes at the back for the heavy duty cable ties to run through and put some padding between the ties and the foam before refitting the seat into the car. Refitting is the reverse of removal but the seat is very heavy and there is very little wriggle room in the coupe. If you can remember to attach the seatbelt bolt before refitting it does avoid a very fiddly job later especially when it is cold, getting dark and starting to rain a little.
The seat is now nice and supportive and I am pleased with the results.
If you look at ISTA the diagram seems to show an SE seat with flex mat under the foam. This is ideally what I would have liked to have replaced but that would have needed the seat to be taken apart and the old one removed from the foam. The German Z4 sites have some in depth procedures to install a modified flexmat from an E46 but I'm fairly sure there is nothing wrong with mine and it looks really fiddly.
Stick Google translate on and these threads are a blast and they have some good photos too:
https://www.zroadster.com/forum/threads/bmw-z4-m-e86-endlich-fuendig-geworden.137835/page-43
https://www.zroadster.com/forum/threads/nachruesten-flexmatten-beim-m-sitz.140930/page-2
https://www.ebay.co.uk/p/1409496225
https://www.zroadster.com/forum/threads/m-sitz-neues-sitzpolster-und-lordosenstuetze-einbauen.135394/#post-2863220
https://z4-forum.com/forum/viewtopic.php?f=2&t=23646 (is a good thread and has a picture of a standard seat with elastic mat and the underside of sport seat)
Pictures from my under my seat.
1. underside of seat pre-fix showing foam in good condition
2. showing the holes I used at the front of the seat frame for cable ties
3. showing the 3x holes I drilled at the back of the seat frame
4. Showing the flexmat lugs into the seat frame. Notice how embedded they are into the foam - how you dig that out to replace is beyond me
5. I don't have 'after' photos unfortunately - but I ran 3 lines of heavy duty cable ties between the holes using 12 cable ties in
