could be the sender unit in the tank - usually got at by lifting the back seats somehow. It could be a switch inside or a cable loose. Prob not your actual guage so - dont go taking it to bits just yet.
You would need to check the resistance on this. With a multimeter the resistance betwen the 2 contacts should change when you move the float up and down.
i have a 1997 r-reg corsa, and when i fill up �40 the max, for about 3 days afterwards it still says there is �40 in it, then after that it will just drop down to empty. i asked the garage about it and he just said it happens alot with corsas, dodgy needle gage. you just have to gage when u think it may run out, by how many miles you have done.