If you're after something quirky and fun but economical, then you could consider a Smart Roadster or Roadster Coupe. Thanks to the small engine they will do 55mpg plus they're great fun to drive - and of course both are convertible (they either have a retractable canvas roof or removable panels). For £7000 you could get the sporty Brabus version which has extra power and a bodykit. Alternatively, there's the funky little Daihatsu Copen which has a neat metal folding roof and also uses a small turbocharged engine so will easily do 50mpg+.