Mine is a 5 speed auto. No matter what I do with it I get a constant 27 mpg which I must say is starting to improve as the engine runs in(current 2k). diesels that I have had previously needed at least 7k on the clock before the engine loosens and starts to give a reasonable return. but I don't think you will get anywhere near 43 mpg.
I've had my 4-speed CRDI for about 18 months, and done just over 20k miles.