Had the same thing on my 420 and now my 620. I kept having to remove the wheel and pop it back into alignment. I have it booked into Caterham for replacement shaft under warranty and I hear there was a dodgy batch! Caterham do not recommend welding it or bonding it. It will eventually sheer the nylon locking nut and maybe the ruin the sensor. It is a pain when you keep losing the odemeter every time the wheels spin too fast and you have to use a gps app as your speedo.