Which grade of fuel are you using?
I've been using Sainsburys super (97ron) for a while but had to fill up at a BP that only had regular, I seemed to go from about 22 mpg to 25 mpg on that tank full which I put down to it being a long journey on single carrageway A roads, I did try 3 tanks of Sainsburys regular and the average dropped to just over 20 mpg but I did manage 29mpg running up to Stanstead on 95ron.
On the trip to Stoneleigh I used Sainsburys 97 ron and managed 28 mpg on a spirited run.
I've been using fuelly.com to record my consumption -
http://www.fuelly.com/driver/talkingcars/zs