well with me, the 40's to 50's is directly on the type of drive... if I'm stuck in a que of tourists (I live in the lakes) and am forced to tootle along at 45-50 mph on the a-roads (carefully feathering the throttle in 6th, trying not to brake or accelerate hard) and this goes on for 15 miles, it'll go above 50 easy.....
If I have the same road to myself, and I'm "having fun" this will drop to 42 ish...
to me, it's just how you drive..... I know if my mother owned my car and was tootling along at 50mph, she's probably have it in 4th gear, and be only getting 44ish mpg..... probably be braking more into the corners, then having to accelerate more out the other side, because not looking ahead and reading the traffic conditions enough ect ect.....
on the m/way for example, try sticking to EXACTLY 70 mph, instead of 80-90, I believe this will save about 10-20% of your fuel on a m/way journey.... (of course looking WELL ahead , and WELL behind to time your lane changes to avoid accelerating and braking)
If you bought the sporting for the "sport" then just rag it and be happy that a petrol hot hatch would only give you 30mpg, and yours is giving you 40