So the Bambers had two quilts/duvets on their bed then Curiosity,one on top of the other?
Each in a fancy cover!
Being elderly, June might have felt the cold in bed even on a Summer night and the quilt could have been rolled/folded up at the foot end just in case it was needed. The bedroom was unheated - no central heating pipes or radiators evident, the AGA providing domestic hot water only.
You'll be arguing next that the police retrieved it from a nearby cupboard and wrapped June up to hasten decomposition! Oops, someone has suggested that already.