Just got back from a trip "down south" in my 1.4 Sport, did 200+ miles of which 150 were at a fairly steady 60mph on the motorway (I did go up to 80ish for a little while to have a closer look at a Diesel Special Edition that passed me!) and averaged 50.8mpg according to the trip computer.
Go up to 70 and you'd have been down to 40-45.
