I have a 59 plate 2 litre TDCi Titanium estate with 48,000 on the clock.
It gets serviced once a year in late Autumn as I do less than 12,000 miles per year. 16 months ago, after service, I noticed that the fuel consumption dropped to the low 40s MPG per tank, but after some months went back to its previous level of around 46-49 MPG per tank (and over 50 on long runs). This year exactly the same has happened, serviced about 4 months ago - poor fuel consumption ever since even on longish journeys. I can't get it above 43MPG however carefully I drive.
I can think of a few reasons, including

• The garage didn't reset something.
• New oil is thicker than 12,000 mile old oil - more drag.
• Winter weather is colder and damper.

Any ideas what might be the real reason?