Agree with martysmartie here,you'll never see the difference with a woman driving it.
My missus gets about 25 out of the ml whereas I've never had it drop below 30 even when towing.
Its a sweeping generalisation I know but women just don't drive economically,leaving it in high gears going up hills,not getting out of low gears round town etc,constantly stop starting rather than keeping it ticking over etc plus their often in a rush so flooring it unnecessarily.
Keep the c-max and forget about the bm,I'd be amazed if you get a straight swap on it anyway plus the petrol fords are really good buses.
One big breakdown on the bm and that's any derv savings gone.
A passat tdi would be a better option with the pd motor,more reliable and economical too.