When my mate first had the problem, I had stopped on the summit of Holme Moss to put my cycling glasses back on (they had steamed up on the climb). As a result, I was doing the descent about 200 yards behind him. He was bombing down at 50 mph but started fishtailing as he turned into that RH bend. I caught up very rapidly and shot past him as he braked to a halt. I waited for him at the junction with the A628 and he eventually turned up, ashen-faced and still shaking! The shimmy had almost thrown him off the bike.
The crazy thing was that it happened to him again on the same event at the same place, one year later! I think it must have been due to the way he rode his bike.
I had no problem at all but I do the
things that control potential shimmy before it can occur (avoid death-grip on bars, press one knee against the top tube, keep my arms and knees bent and raise my weight slightly off the saddle).