If the car is anything like due for a FULL service, that alone may be the answer to your problem.
High fuel consumption is generally indicated by BLACK exhaust smoke, particularly whilst accelerating - have you noticed this?
High fuel consumption can be caused by either too much fuel being delivered to the engine, or too little air entering the cylinders.
If the servicing IS up-to-date, with the above in mind, it is worth checking that the problem is not being caused by something simple, like a plastic bag having been drawn into the air cleaner.
Are there any other symptoms? eg does the car drive at 'normal' engine temperature?
If it is running cold, the thermostat may need changing.
If the above mentioned do NOT apply, I suggest that you get any fault codes read and cleared (deleted).
Then drive the car for a couple of days and re check for fault codes, it is likely that any new codes with point to the problem.