Having studied numerous pictures of C15s, it seems my routing of the speedo cable (around the side of the headstock) is wrong. The original routing appears to be up the downtube and then looping under the centre of the bottom bracket with entry between a slot in the front of the bottom bracket and the rear of the nacelle.
I have tried this and it certainly looks much easier for the cable. But - I cannot persuade the thumbscrew fastening 'nut' on the end of the cable up through this gap. It crossed my mind that maybe the cable should be threaded through down from the top - but the speedo gearbox nut is even bigger, so that cannot be right. I guess I could release the nacelle fastening screws a little (and risk dropping them down inside the forks - again!). But I am doubting that BSA made swapping a speedo cable that involved...
So does anyone know the magic trick?
yes - in front of the lower yoke and behind the front nacelle - sandwiched between the 2
I just removed the 2 lower yoke bolts and jiggled the thumbscrew up - at the same time as teasing the tin nacelle out gently with a flat tyre lever - super gently
Worked a treat. Thanks!