My 1.9tdi vw lump won't do 70mpg no matter how carefully I drive it, 55mpg is a much more realistic figure!
Obviously a mini may be lighter so that would improve things a bit, but if you're going to the effort of producing a 200 bhp diesel mini you're going to want to get your foot down which may reduce economy further!
my vw pd130 motor has only ever returned 58MPG ( via the optimistic trip computer ) and 52mpg by working out the MPG manually. They are good engines, but not as good as people make them out to be.