I'm not sure how to go about this. If the sturmey archer mechanism is the same as a standard gripshift then I'm supposed to shorten the cable from the hub end and not the other way round. However, the 'cable connector' at the hub end is attached to the cable via a simple crimped piece of metal that effectively clamps the cable. So I could cut that off, buy a replacement and use pliers to crush it onto the cable, but I'm not sure a that'd result in the necessary amount of friction. It looks like a special tool has been used to crimp correctly.