Don’t know if this is any help but I’ve had the same issue with my Foggy FT and have had the first service done and discussed it at the time with Ben, the mechanic. He backed the suspension off at the front for me and I didn’t notice much difference. I then put it back myself to where it was and notched it up even a little more and again, I didn’t notice a right lot of difference.
I’m 5′ 10″ tall and weigh c 11 stns 7 lbs and have come to the conclusion that it’s me and my riding style. I am used to adventure style bikes where sitting upright seems to be the order of the day and you can’t reasonably do it on the FT, as it lets the bars bounce all over the place. I find that when I lean forward a little and put more weight on the bars (not as aggressively as on a sportsbike), I seem to be able to eliminate the wriggly head, and have not managed to induce a tank slapper since!