Just a follow-up note to add to this puzzle. I've just found out that Alexander Keiller who was heir to the Keiller marmalade fortune, wasted much of his inheritance on failed business ventures, one of which was a system of car seats which folded down to form sleeping arrangements.
Obviously this may not be his work, but it would be the correct period, as he was active in inventing useless things between 1920 and 1940. Grasping at straws, the tartan seats in the photo would reflect his Scottish background. Although this Hillman was not on the list of cars being exhibited, does anyone have a list of the coachbuilders and accessory stands?