Thread dredge. After sitting in the garage for two years, I just got this sorted. The alternator was a Honda one - and Hondas spin the opposite way to Toyotas. The new regulator had accounted for this, but not for the fact that the internal fan blades were orientated the wrong way around and so not cooling the alternator. Was cooking it. New alternator and I've put about 500km of round town driving in without an issue.