Holy crap this thing is running better. It was a hard failure that left the bike unrideable. But it also looks like the failure had been coming on for some time. She pulls the way I remember her pulling when I bought her not that many miles ago. Woohoo!
Of course, this means that just to carry out a complete detailed confirmation of the effectiveness of the repair you'll have to keep going out riding at every possible opportunity for as long as possible. It's a dirty job but...