A bit down on MPG

[The Aardvark]

Hi All

Just had my BMW E39 530d serviced (Inspection 2) and water pump replaced. Seem to be getting approx 15% less MPG according to the onboard read out (not saying that the 2 are connected) - anything I should perhaps be looking out for / checking?
Appreciate that economy is down during colder weather, but even during this recent milder spell I'm still not returning what I used to achieve.

Any thoughts?

[Petrolhead_Rich]

Different fuel to usual?

Different Driving Style?

Wrong Air Filter fitted/not fitted correctly so too much/not enough air is getting in?

Sensor disconnected during service?