How can i fix my fuel remaining reading?

I put petrol in my 2003 renault clio but the display which shows you how many miles you have left til empty didnt change. the needle moved slightly but not a lot (ok I did only put a fiver in). the warning light wasnt on yet.

I figured ok I didnt put enough petrol in, so I did a few calculations (£1.25/l = 4 litres at 45mpg) put another fiver in and still nothin, it still went down at a standard rate and in fact cleared the display altogether once it got to 45m left til empty! The light is on but Im certain there should be at least 50 miles left!

Please help??!??!?!??!??!!?!?!