The break in the inner cable has been caused by damage to the outer, by what means I know not. Having proved that the inner cable was free from the speedo to the break point, inserting it from the gbox end met with resistance at the damaged point; so any idea of cheating by simply replacing the inner went straight out of the window! However, trying to extract the outer cable met with strong resistance and no way would the connectors at either end clear whatever was holding it. OK, I cut off connector and pulled, and out it came. Fine, now just re-route the new cable and all will be fine, right? Wrong! No way can I find a path to the gearbox drive. Looks like a while before I am back on the road.
Meanwhile, I have replaced all thirteen instrument/warning light bulbs, lots of fiddling but great result.