I was getting the same approx3 2mpg during the week when i only travel 5 mile to work and then 5 mile back.
on a week end when i go further i can average about 45mpg. Motor way travel works out at about 50-70mpg at 70mph.
i was not happy with my milage at a short distance either so i purchased some magnets of
that go on your fuel pipes as close to the engine as possible and now insted of 32mpg i am getting 41mpg. I went a bit over board and added 6 pairs of magnets but it does say just add 2 pairs for diesels.
It has also improved my performance slightly as well.
Follow the link below.