I found these posts as I had exactly the same issue and was looking for some advice!! If it had been in my own home, I would also have considered adjusting the supply pipe. However, I was doing a favour for our local Community owned swimming pool - so really, really didn't want to start altering the existing pipework!
After a lot of thinking, I decided that the problem was the gap between the top of the black sealing washer and the bottom of the fill mechanism. The washer needed to be snug against the bottom of the fill stem AND fit into the hole in the bottom of the cistern. This wasn't happening as the brass shank was too long and when fitted into the inline valve below the cistern, the bottom of the fill mechanism stem was about half a centimeter off the bottom of the cistern. (I measured the gap very roughly by pushing things like a pencil & allen keys (!) into the gap to try and gauge it!)
So, I took the black washer off...and put on 5 or 6 large white plastic washers, then the black washer on top of these. In effect, this made the fill stem push on the washer. I put it in - without much real hope - but, blimey, it worked!! I only had to do the bottom nut up hand tight - and nary a leak was seen.
I am rather pleased with myself - and thought I'd share!