vauxhall astra 1.6 2004 model, petrol problem!?

My petrol gauge gets down to a quarter of a tank and it behaves as though the car has an empty fuel tank, the car will then run out of petrol even though th gauge is showing a quarter of a tank, I have never seen it go below the quarter mark and Ive never ever seen the petrol light come on to warn me, probably because it never makes it below a quarter..why is the fuel reading wrong and what can be done to correct the issue? This has been inconveniencing me and driving me nuts for 8 month! I dont want to carry a full petrol can around in my boot for the rest of the time that I own the damn car! All advice, any advice is very most welcome..Thanks in advance...:-)