C5 Mk1 Driver's Seat foam Cushion?

[LincsCitroenC5]

We have a 2002 C5 2.2 HDI SX. The driver's seat base cushion has collapsed. The driver has been pumping up the height adjuster for a few years to compensate, but that has reached its limit and the seat offers no comfort anymore. The car is tidy and only has 120,000 miles on it, but the seat is making it almost unuseable.
The spring base is fine so can anyone please recommend somewhere that can fix these cushions or if we'll have to hunt for a used one, etc?
The Citroen dealer thought they could get a new cushion at an eye watering £430 but didn't know if the cover was part of it or not (although it isn't available separately).
Any help is appreciated.

[GiveMeABreak]

Pop your VIN up if you want the part number to aid in your search - they are all different depending on equipment and upholstery.

[wheeler]

The cushion & cover are normally both seperate items

[GiveMeABreak]

Not on these - it is a complete part for the seat bottom, in 11 variations depending on trim, referred to as seat lining cushion.

[Hell Razor5543]

I can confirm (through experience) that the cover and the cushion are one item (as the cover is glued to the cushion).