my jag xf 2013 had a service recently, and since then my average mpg has dropped from 55 to 35

 

Too few car details.

The answer is probably to take it back.

 

Ever since yall did my service my remote dont work,
ever since yall did my service my seat heater doesnt work.
Ever since......
Theres a whole website with all this stuff
and 55mog? You must have a diesel cause theres not a gas jaguar that even gets close to that, but really what do you think they did on your oil change? Flip a switch to cut your mileage figures down and double your horsepower...

 

Have you filled the tank and reset the trip?

If it was worked on with the engine idling etc, the mpg would drop on the trip

Otherwise, they may have flipped a switch