My 60L tank took 59.8 litres from empty yesterday - according to the pump. 
So use a different garage, as that one is conning you. Unless you haven't got your tank right.
Tank is too small. IME, the more you fill each time, the more economical it is. I reckon a set amount is lost with each fill.
This was the new BP on the A12 opposite Colchester football ground.That one varies enormously for some reason.Sometimes it cuts off before I think it will,other times it does what it did yesterday.I find many of the pumps round these parts will put well over 50L in mine (according to the pump) the only exception is one near where I work,but Ive just had a week off and ran out on the way home from Chavtown on Sea yesterday.
I have considered a bigger tank,it would be nice,but too much aggro to get hold of one and fit it really.Maybe next summer,if I still have the car I will look into it a bit more seriously.