Steve,
I have a Luxury spec, with nav and full leather spec (heated and ventilated seats.....already showing the benefit with the temperatures in the high teens over the weekend). I an going purely on the onboard computer, and have not independently verified it. I am afraid I am not one for logging every time I fuel up, as it will not change the way i drive knowing that one particular fill up gave me 0.3mpg more than another.......Work on the assumption that the onboard computer is as about accurate as every other manufacturers', and as a gauge, was getting about 49mpg from a Astra 1.7CDTi that I had on hire beforehand, 41mpg from a hired Astra 1.6 before that, and about 50 mpg from my company Passat 2.0 TDi that I had before that. None of these, however, had the power, the poise, the plushness, the specification, and more importantly the fantastically low company car BIK level that the IS has.