The Argos catalogue has a useful guide to selecting a sleeping bag. If you've got a copy, it's on page 120. Otherwise, here's a summary.
First, choose the correct tog rating. Recommended tog values are as follows. (NB: The temperatures are, of course, night-time ones!):
Summer (15C down to 8C) > Tog 3 to 5
Spring/Autumn (10C down to 0C) > Tog 5 to 8
Winter (3C down to -10C) > Tog 7 to 10
Mountain (-5C down to -20C) > Tog 9 to 12
Polar/High Mountain (-15C down to -40C) > Tog 11 to 16+
Then choose the shape:
Envelope bags are the basic rectangular type.
Cowl bags retain the rectangular shape but add a 'hood' to keep your head warm.
Mummy bags are similar to cowl bags but the shape is designed to fit your body and retain maximum warmth.
As well as providing useful information, Argos offer quite a wide range of sleeping bags, at reasonable prices, so it might be worth taking a look at their range.
Chris