Faulty sensors/MFA unit?
My MPG readings are very low aswell although the actual petrol consumption doesn't seem too bad. Not too much different to my mates Astra GSi.
Can't find any local scrappers with gti's in stock still containing the MFA parts and a repair looked to be quite costly so I just don't bother with it. Only use it for oil temp, journey time, distance and average speed.
Ian