Interesting.
My car, on a 50 mile round trip mway commute used to do 36-39mpg all day long.
In January my commute changed to a 12 mile round trip consisting of 5mph traffic and then a short blast on a bit of mway and a road. I also had the timing belt and everything else changed around this time too. This has seen my mpg drop to the 27-30 bracket quite consistently.
It's obviously hard to tell, due to the massive change in journey, but I had assumed that my timing may have been out before, or that it was just the journey itself.
I may look at changing the front lambda just to see if it fixes it. How much are they and where from?