Hi Juggy1985,
We recently picked up a 2009 LCI E91 320i Business Edition M-Sport - see my post
here. (Pictures of the car we bought are on page 3).
I can't comment about the 318, but this release of the 320i does have cheap tax (£130) if that's what you are looking for. MPG wise, it all depends on what sort of usage the car gets. Our car see's mostly short journeys around town; school runs, Tesco's etc. With these we see an average of 26/27mpg. On a trip up north we were seeing 50+mpg at 77mph on the flats, but still averaged out at 36mpg for the whole trip.
Size wise, the car isn't that big. In fact I am not sure the boot space is really any more generous than our old Focus. If this is your main requirement, I would look at another car!
If you have any questions feel free to ask.
Cheers
Mark