Yeah, I honestly think sub 300 miles is nearer the mark for an average Cooper rather than the 400-450 you are trying to achieve.
I know its difficult with petrol prices racing up as they are but I try and forget about MPG and drive the car as you want to, cos soon it will be too expensive to drive anything other than a 1000cc eco car and you'll regret not driving the car to the max when you had it

!!
Rich.