Sounds very frustrating !!
Not sure if it's been mentioned before but Patrick had a problem with pin holes in his Marlin supplied tank. (he had them welded up)
In the long run it may be time to remove the tank and inspect all the welds closely under a strong magnifier and bright light?
As an alternative to welding, there are tank sealers about like Slosh (
http://www.rust.co.uk/products/produ...rol-tank-seal/ ) but I have no personal experience and I'm not sure if they are suitable for an aluminium tank. (but one below says it is)
Some other products here...
http://www.caswelleurope.co.uk/gastank.htm
and here...
http://www.frost.co.uk/por15-fuel-tank-sealers.html
and here...
http://www.tankcareproducts.co.uk/
I hope you get it sorted soon as I bet your other half is getting a bit miffed!