have you car checked on a gas analyser, to check co% content, if running rich could be thermal time switch or another sensor or air mass meter any one of these could be sending incorect info to ecu, which would result in fueling to mutch,whats the temp gauge running at?, this time of year,
should be just above half way, if under this engine not running at temp, so again ecu could be overfueling,
10mpg seems rather drastic... a couple of MPG could be put down to it being colder, running the blower, rear window heater, having the headlights on , and so on.. I find my 2.3 drops by about 2-3 during the winter but if my average mpg went down by that much I'd be concerned.
This might sound silly but is this going purely from the diagnosic computer or is this real miles on the clock and fuel put into the tank?. Make sure that you haven't managed to switch from MPG IMP to MPG on the display... I did that once and it dropped quite a lot.
Some further suggestions assuming one of the above suggestions does'nt do the trick!
Check tyre pressures. The cold external tempretures can make tyre pressures drop significantly (if they were filled with warmer air on a warmer day!). Low tyre pressures can knock a few mpg of the norm.
Also, short journeys will show a large reduction in mpg on a cold day - as the engine takes longer to warm up, but 10mpg down on good runs is definately alot.