Thank you all for these suggestions, Ill check them all and will let you know.
The car is a manual estate with 16-17 inch rims. I think my fuel tank is 47 liters up to the point which the fuel warning light come up. Using this reference I checked the mpg only over mostly motorway driving at 70 mph using 20 liters of tesco fuel this time and I got 179 miles or 40 mpg which is quite better. I refilled the tank once more and will check the usage again using tesco fuel.
If any help, the car has one more issue too. That is after 30 minute of driving, when coming to halt or slowing down , placing the car into netural the car idles high momenterly above 2k before falling back below 1k. Down shifting helps a bit