I agree with the majority... It's likely to be standard servicing, tyres, plugs, air filter, oil & breathers. Even if none of these look drastically bad, improving them all together should make a difference of some sorts.
Other things to check are the timing/mixture. Engine temperature is a good point, a warm engine is an efficient engine as my ol' man used to say

If they throw up nothing then you'll have to do a bit more digging... brakes binding and other stranger things.
Only other thing I would add is bear in mind you can loose about 6mpg in the cold weather! And with a V6 I would 25mpg was a very acceptable figure. My 2.5 Auto used to dip to 18mpg unless I was meticulously careful