The Sleeping Bag question comes up pretty often, and depends on how you sleep (hot/cold) and price. By the sound of it, your getting a miserable nights sleep, and since a 3 season should get you down to about zero comfortably (you hope), your probably sleeping cold. You could go for a fleece liner (about a tenner if you look in TKMaxx), but a new bag seems a better bet.
Personally, I'd get the warmest bag I could for my budget. A quick look on Ebay brings up surplus 'arctic' bags from £10, although £40 seems more average, but they will big relatively big and heavy. I've got an old (very old) Snugpak Osprey (which isn't bad, but Snugpak bags have the reputation of being optimistic in their ratings), but when I can afford to change, it'll be a down bag (less bulk, weight and a lot cheaper than they used to be). You might find one cheap on ebay, but realistically you paying about £150. But for £140 you get the highly rated Alpkit SkyeHigh 800
http://www.alpkit.com/shop/cart.php?target=product&product_id=16321&category_id=286, which is the one I've got my eye on. They've got a good guide to buying a bag
http://www.alpkit.com/spotlight/choosing-a-down-sleeping-bag
If you want a surplus bag, ring Kevin at Endicotts. They don't have them on the site, but I'd be surprised if they couldn't get them, and you'll know you'll get top kit from them. If they quote a price/grade, then that is the price you should be paying for it.
Also think about your sleeping mat - they can make a real difference.