In reply to RKernan:
Sleeping bags are an odd item to buy - above £150 price generally relates more to the brand name than the bag, its easy to waste a lot on a little.
With cheap bags its the opposite, you can pay little for a bag that promises a lot, but doesn't deliver. Generally these bags use either cheap low quality down, or cheap low quality synthetic insulation, and do not wear to well. Ideal for car camping in summer but generally you won't get much more under ~£75.
If you max budget is around £80, look at the Mountain Hardware Lamina range (I'm sure you can get one for between £80 and 130 depending on the temp range you require)- they are an absolutely awesome range of laminated synthetic bags. Low weight and bulk, hard wearing and really will deliver in even the worst of conditions.
Hope this helps
Ben