El Saxo

Personally I'd say go with a 'normal size' 2/3 season sleeping bag. I got a 1 season Vango thing last year that was only 15 quid and packs up so small it fit into my tankbag along with my wash kit and a few other bits n pieces, but on more than one occasion it was bloody freezing, even with an extra liner bag inside it. A lot of the larger ones will still pack down fairly small by using the straps on the outer bag.