My gauge also fails to get to the marked "full" too.
However, the exercise did give me a clearer safety margin when low on fuel. Having seen the gauge with my estimate of 10 litres left it gives a better safety target.
I was initially thinking of removing and flushing the tank over next winter. If the troubles return I will remove and possibly look at a new higher capacity tank, but not certain yet.
One definite move is to instal an extra in-line filter between tank and pump - added filtering before any "working bits " .
