Fuel filter question

Hi, all I'm wondering if someone can help me on here had the ep3 for a while now. I just dont understand why my fuel light comes on with 15 litres remaining on fuel the most I've filled up is around 35-37 litres max and it's a 50 litre tank. Anyone else come across this. The filter was changed with genuine one at 90k cars now on 120k also I noticed a seperate strainer is sold doesn't this usually come with the filter. The only thing I can think of is which I've been told because I've had it fitted from a local garage maybe someone has either bent the float or not fitted properly but other than early reserve my car runs fine the gauge is working fine also unless it's the sender unit not sure. Anyone have any ideas please as I'm not sure where to start.