That's shocking mate, you must be driving the tits off it.
Mine is completely standard, I average 35 - 39mpgs but usually see 400miles froma tank before the light comes on. With the temperature drop I'm not expecting to see more than 360ish - I do give it some beans too. I usually (from brim fill) see around 230 - 250 miles from the top of the tank alone. Short journeys, start stop traffic will of course effect this. My daily commute is 21 miles and I average 50mph on A roads.