I studied some pics of choke cables and reserve tap cables and measured the size of the mount just behind the knob, and guessed that 3/4 would be close to the nut size. Then I got lucky and found a tube spanner of 3/4-5/8 on a shelf. Cut in half, slotted, bit of filing to get the slot just big enough to go over the cable. Bingo, it fitted well enough to tighten up the nut, now all works as documented! Smaller hands and better access would make this a lot easier, but its done for now.
When I looked up the tool in the WSM, its described as suitable for the choke and 'reverse' cable nuts....