Which engine does the Mondeo have in it & what is the age.? Many years ago i had a 2.0GLS Mondeo Estate that was also thirsty. I had the Lamber sensor changes & this made a slight improvment to the MPG. Give the car a good service. Change the fuel/oil/air filter, change oil, change spark plugs. Check your tyre pressures. Use 5-30 semi synthetic grade oil. The Bosch type plugs appear to run better in Fords.
Richie.