As i said matey, i set the tank spot on with a angle gauge, so i think that bit is fine.
OK. In that case, you said you put in 15 litres at first. Presumably you then burnt some of this and filled it up again until it stopped filling.
I would suggest that either the stop valve is cutting out a bit early and not letting the tank fill to 80% (as I said, i believe you can adjust these on a multivalve) or that the fuel pickup is not quite reaching the bottom of the tank so it didn't completely drain the tank before you refilled it. This would tally with your expression of surprise that the 15 litres didn't last long.
Do you have a manual gauge on the multivalve? Have a look what this indicates when the tank is empty (switches to petrol) and full (filler cuts off). IME this is a reasonably accurate indication of the state of the tank, whereas the electronic gauge needs a bit of fiddling to get it reading right.
I wouldn't necessarily take what the pump says as gospel either. When I first filled mine, it took exactly 64 litres, as expected. I have subsequently refilled it from a state where the engine was still running on gas and it took 67 litres. I believe there is some temperature compensation in the pumps which means you get charged based on the density of the fuel not just the volume. Having said that, 52 litres is on the low side.
Kevin