The Snugpaks are exceptional quality. They also seem a bit more honest than other brands on the comfort and extreme temperature ratings. I have an Autumn bag which has kept me toasty in the Peaks in November.
You can get Kilo bags for about £50 +postage via ebay.
As my Autumn bag is too warm for summer I also have a Blacks duck down bag which cost about £50 IIRC. Suprisingly good quality, very light and compressible. Rated +3 to 10 comfort, -5 extreme.