For anybody else interested, the adjustable backrest from the Amazon link above seems excellent (InTexas kindly lent me one to try). Sturdy, quite lightweight, doesn't slip about on the bed, and less than twenty pounds (couldn't find it cheaper elsewhere, though of course I will have to buy something else to get free delivery grrrr). It is quite large, but provided I can find somewhere to store it when not in use, I will be buying one! The cushions are fine but the backrest definitely gives more support. ETA: The backrest fits under the bed when folded! Hurrah!!