When i first got the car i was getting around 17mpg, as the lads said if you boot it everywhere this is what you will get. however, i get about 20 now, have had 29mpg with "sub-urban" driving so it can be ok.
Agree with chris though, mine has been noticably lower in the cold!! Generally its to do with density of the air and the fact the engine takes longer to warm up, obviously using more fuel, hence why you use less fuel in the summer.