It had actually broken just where the cable tie had crushed it against the front panel. Further along was another tie that had also crushed it; I chopped it there and spliced in a new length to be on the safe side. I'd say the crush force had 'smeared' the insulation, exposing the copper which then corroded through - it was nice and green and crusty which is the usual sign of oxidised copper.
Incidentally, returning to the 'WTF' in the thread title, what I meant was I couldn't understand why they did such a simple job as driving an alternator in such a convoluted fashion. However now I think I understand it's to do with the fancy charge control system and allowing the engine to 'over-run' the alternator so the alternator doesn't cause unnecessary drag... or summat
See, when I trained as a motor mechanic cars still had points and drum brakes...

Connect-ed since 2003... now in a 2014 T240 115PS Ltd.