I guess the gearing is changed in the traditional way, by someone with a stick moving the belts across to the next diameters along. Apparently that used to be the emergency stop in the old days, too, some bloke poking at the belts with a pole until they fell off. Probably not ideal if it was to stop someone being sucked into the mechanism...
GPS Coordinates:
51.44632, -2.61726
Location: Baltic Wharf