I had this problem, stripped, checked, its not leaking from the seals or casting it appears that its leaking out of the hole in the side of valve !
So a new valve should fix it, I have heard vw fuel valves are the same and cheaper ? But this small pressed steel valve is made by Bosch and is clearly maked up with parts numbers etc so should be easy to buy, so this is what is foxing everyone the leaks are not from the seals but from the body of the pressed steel valve so something's gone in it, hence all these attempt at seals be replaced are a waste of time just replace the little valve leave everything else alone ! By the way I didn't even replace the gasket as the old one seals up fine and all the o rings were ok.