Good stuff Andy. The old Airfix FBVI kit had a similar shaped seat base, but the back rest cushion is more rectangular. From what I remember of the (now sadly lost) BAe TIII, the cushions were not far different to those in the Airfix kit, and the Revell is more or less acceptable.
On the pilot's seat, that rounded moulding on the armoured back rest was a leather cushion, the edges of which were 'rolled', rather like rope, but also leather. The colour was normally very dark brown.
Beneath the pilot's seat there was a stainless steel 'relief tank', cylindrical in shape, with a hose running from it to a clip beneath the forward edge of the seat, the end of which had a stainless steel 'cup' shaped like an elongated wine glass. At the side of the tank, facing starboard, was a First Aid kit, normally rectangular, around 12 or 14 inches wide approximately, painted white, with a red cross centrally located.
Both of these items can be clearly seen when viewed from the lower hatch, although the First Aid kit blocks out most of the tank, with mainly the hose and cup visible.