Hi Patrick,

If you had the air filter changed and this increase in fuel consumption happened straight after, it is very possible that the wrong or a more restrictive air filter was used. That would be the first place to start. 

Other than that, check your tyre pressures (very important for fuel economy). And of course, as it is moving into winter, it could be that you have the car heater fan and rear demister and wipers and lights on more, which all have a detrimental effect on the economy.