There's a couple of things it could be: it's either the gauge (wiring poss) or the fuel sender. The fuel gauge is basically just showing the signal from a variable resistor. If you can get at the wires, you can test it by grounding the wire. If flips when you ground it, the gauge is ok. Then it might be the sender, which I believe is accessible from the boot of the car....some cars have the sender in the back wheel well, and it'll be sealed in with liner to stop water etc getting in.

I'm not sure on your model of Fiesta; I think they were under one of the seats.....

I will have a little search but I have a manual for older Fords unfortunately.