Grey Steelcase Leap task chairs (CE)

Dark Grey fabric.
Height adjustable base.
Fully synchronous floating action.
Seat slide, adjustable arms.
Flexible backrest ensures that the back is always fully supported. (See picture below.)
The separate upper and lower back controls can be adjusted to provide full support to any user.
The dynamic seat takes the pressure off the lumbar vertebrae when reclining.
Sustainability: Leap is 98% recyclable and contains no hazardous materials.

One of the best chairs on the market.  Would have cost about £750 when new.