You can quickly fix that with a simple dot of any glue (pattex, Loctite, whatever you want). Just pull of the knob, place a tiny dot of glue on the switch's shaft, let it dry and then put the knob back to it's place. You'll never have the slipping problem again and the dot of glue could be removed easily with a screwdriver or a knife.

TeeMcBee
Got my Mk2! # 080000143