ha ha! Your fuel tank is an irregular shape so the gauge doesn't accurately indicate anything. Your engine can't really turn fuel into nothing without other symptoms. Wait until the gauge is at zero, brim, the tank, then recalculate. I know that's a long wait, but if you start trying to estimate a 1/4, 1/2, 3/4 tank and extrapolate you'll end up with bipolar results. I know-I used to do it!

You can make sure that you don't have any dragging brake discs or a handbrake that isn't releasing properly. That could take a chunk out of your fuel economy.

OP - you have used some very arbitary assumptions and guesstimations to try and prove your point that you are getting poor mpg.

I'd expect that car with a 50 litre tank to get a minimum of 400 miles between fill ups. My dad used to get 45mpg from his Fiesta 1.4TDCi (same engine) when teaching driving.

Thanks for everybodies advice, I am pleased to say that after doing another 100 miles today, the fuel was between half and three quarters, I thought I would fill it up again. I was so relieved that it only took 12.5 litres, with a total milage of 150 miles, so about 55mpg, guess what though the fuel guage still hasn't gone back up, I guess I have a problem with the sender unit. Is it easy to change this, or do I need to take the tank out?

Thanks again for all your help.

PS I checked the brakes as well in a very primative way. I took the handbrake off and the car rolled very easily with a push, so they can't be binding much.