I think you may have to buy some new wellies! The trick for streching boots with wet paper etc. only works for leather boots. Leather can be coaxed to "give" a little bit (i.e. stretch) and will stay stretched. Rubber boots will stretch a tiny bit when you put your foot in them, but like a rubber band they will snap back to their original size as soon as you take your foot out, and I don't think there is anything you can do to change that. Keep them as your summer pair and buy some more for winter. My dad used to wear his cheap and cheerful sloppy wellies with oiled wool sea-boot socks. Nice and cozy!