Re: The Puma and SEAT.
Probably not entirely wise deciding the Puma does the same MPG as your 500 as a factor in wanting one. They aren't uneconomical but the fact one guy is getting what you're getting from the 500 means one of three things:
a) He's driving like a hypermiler and you're getting respectable economy
b) He's getting roughly what a top percentile of Puma owners get and you have a bit of a leaden foot in your 1.2 Fiat.
c) A combination of the above.
I don't know what either of you are getting but picking a figure out of the air I'd expect a Puma to average around 35mpg in the hands of most people, perhaps a bit less if you use the performance. I'd expect that Seat TDI you're also looking at not to get less than 40mpg unless you drive it like an utter prat (which, let's face it, is how most people drive VAG diesels).
The Puma really is a cracking car though if you find one that's been looked after. Some earlier ones can use a fair bit of oil as the Nikasil bore liners wear away (just like they do on Jag XJ8s and Porsche Boxsters) if they've not had regular oil changes and they've done lots of short journeys.
The SEAT would probably be a more pleasant car to own but not as good to drive. More space, more economical, more relaxed on longer journeys, and probably a smidgen more reliable.
Keef - some of our small, crappy cars would probably surprise you a great deal. There's a reason why we love them over here (some of them are great) and there's a reason America doesn't like small cars that much (you rarely ever get any of them which aren't utter crap).