I have a similar car, although a cdt, not the 'i'. The fuel consumption does vary a lot. Its the town traffic that does it. On motorways, according to the display, at 60 - 70 it shows about 45mg or higher, (that is setting the display at that speed and not counting accelerating up to it etc). I have just done about 240 miles, mainly A roads and dual carriagways, and about 20 miles round town (but not central London) and my dash display is showing 37.9mpg.
On long motorway runs I have had it average over 40mpg. It think it depends when you reset the display the period it calculates over.