This is a dilemma I've had on trips , a good 3 season sleeping bag in that size that wraps up real small would cost a fortune . In the past I've settled with strapping my sleeping bags ( two of them ) on the top of my panniers . I've since done some research and settled for OEX Roam 300 , this is a good 3 season mummy style bag that comes with its own waterproof bag that wraps up nice and small, British made also..! . You can get them at Go-outdoors at around £45 .