Have you noticed any black smoke when Accelerating? check the tail pipe, is it "Sooty" with respect to Tony's answer I think you have a pump fault, on 80000 it should not need the use of Injector Cleaner, the car should return at least 50+ MPG, Is there a roof Rack on the Car? what gears are you using? do you get into top gear as soon as you can? If there's a previous owners name on the V5 contact them and ask if the had the same issue regards the MPG, the car needs to go onto a Diagnostic machine that will tell you how the car is running, have you contacted the seller with your concerns?