I could make a sort of quilt out of pertex and thinsulate for £20, much cheaper than a poncho liner. I'm not sure my sewing skills are up to that though.

On saturday I bought a pretty large nylon stuffsack which weighs nothing for £2. It has a handle across the bottom and a long drawstring. By tying the drawstring to the handle, it can be carried as a hudson bay pack. This would work perfectly for carrying my bedroll.