The most likely cause would be the rubber seal on the fuel cap. Often these are not fuel grade rubber, become contaminated with fuel which causes them to swell, which in turn can allow water ingress. I'd start there, and then look for other causes.
This was my first point of call when I had the original problem and the rubber seal was changed .There was no tell tale evidence of water getting past the replaced seal even if I played the hosepipe on the cap having first stuffed kitchen roll in the filler pipe . I will retry again tomorrow just to reconfirm.
The only other way in is via the tank vent but this is a proper fitting and angled slightly on the hull It is very unlikely that large amounts of rain water can find its way in although it is the ideal route for hydroscopic absorbsion from the very wet atmosphere we have been experiencing recently.
Bilge is dry, Fuel Tank surface is dry and it is fresh rather than salt water that is being collected.
Obvious answer is.........measure the amount of fuel left in the tank accurately (assuming the 10 litres is an accurate measure of the amount of water removed), if you're left with 40 litres of fuel, you are correct that is is an ethanol issue, if you still have 50 litres of fuel you have a water ingress issue, simples I think!!!
Good suggestion Rob ....Only problem is where do I store 40 litres of fuel?....Looks like I will have to buy more Jerry Cans I already have 2 filled with fresh fuel I was going to top up with today.
The Guys From Holes Bay Marine were saying they are seeing a lot of fuel separation /water contamination issues currently and also thought it was due to the ethanol content of the fuel .and the fact that I am not using the boat often enough ......I WILL GO THROUGH THE FUEL SYSTEM WITH A FINE TOOTH COMB TOMORROW AND HOPE I CAN FIND A WATER INGRESS POINT ....
Alternative is to pay someone to take the boat out at least once a week....any takers??
Thanks For the advice guys.
Peteg
More cans Pete!