Following on from the Great Crank Debacle the same pal phoned me up in a bit of a sweat.
Had a Boardman carbon framed road bike where the rear mech had stopped responding to the Tiagra STI shifter. The shifter clicked but the cable did not move.
Spotting an opporchancity to redeem myself/show off to girls I headed over and began diagnostics. Five years since I touched an integrated shifter so fired up Shimano tech docs for the exploded diagram based on the product number. Caution for once.
Deduced that the cable end had come adrift from the slot that connects it to the shifter ratchet. Not possible on an MTB shifter of course. The cable end was wedged in the run that goes over the top of the shifter before turning round to head backwards onto the bars.
Bar tape off, shifter off the bars to get proper access. Multiple attempts with improvised tools to get hold of the cable end. No joy. It won't move backwards or forwards. Cutting the cable will be fatal.
Consult with pal. Tell her what will happen in a bike shop if she decides to hand it over. Tell her how we can fix it. She decides to allow me to go ahead. Here we go.
Consulted the exploded diagram. Drilled into the side of the shifter to expand the slot through which the cable end is visible. One millimetre at a time. Easy does it. Eventually the cable can be pushed out and replaced in its slot.
Bar tape back on, hoods folded back to hide the drilling we have a repair.
Phew.
Root cause: Halfords mechanic hadn't engaged the cable in its slot properly.