If your just looking for cheap and cheerful then i recommend a good condition 90 pattern bag and the issue 3/4 length inflating mat. The bag can be had for about £20, the mat the same price. If you factor in another £20-30 for a bivvy bag in good condition then you have a system that will keep you warm and dry all through the year, although it can be a bit warm in summer, and the bag is a bit bulky, but bombproof.
If you want more comfort have a look at the alpkit dirtbag mat, it's much thicker than most and has a taper so its a bit thicker near where your head would be, very comfortable and actually doesn't pack down much larger than a normal full length mat.
If you would like a new bag then i can recommend either the snugpak softie 10 or a bag by Vango. Vango are a good budget brand and most shops carry them so they're easy to try out for size and fit. The snugpak is the best of the range I've seen, but the zip is a bit poor, and its a bit tight across the shoulders. In general snugpak are optimistic with their ratings but the 10 has kept me warm in some very bad winter weather.
Another option to look into would be the mountain hardware lamina range. I've been eyeing one up for ages now as a replacement for the softie 10 and going by the specs and from what I've heard from others who bought them they're excellent bags and true to spec temp wise. They run at about £140 for a 3 season bag if i remember correctly.
Tonyuk