Hi @Steve vialls there are lots of possible causes, it could be as simple as low tyre pressures or your driving style. With the lockdown going on most people aren't driving as much, town mileage won't be as good as motorway mileage. Could also be bad sensors, bad MAF, bad injectors.. an endless list.
If you have access to a scan tool it may be worth scanning the car to see if there are any error codes stored in the computer.