Finally got a chance to give the Activa some attention today.
It has been having issues in that the aux drive belt is determinedly doing this:
You can straighten everything up again, but the moment the engine starts, it's riding off the edge of the hydraulic pump again.
After quite carefully checking that everything is rotating smoothly, there's no play in anything, the tensioner seems fine etc, everything is sitting square.
However this looks odd to me. I'm pretty sure there wasn't a shiny edge on the crankshaft pulley originally...
What I'm thinking at this point is that the rubber joint between the two parts of the pulley has failed, allowing the outer section to slide towards the inboard side of where it sits...and that's pulling the belt with it.
This seem a plausible explanation?
If so it's probably less of a pain to sort than changing the tensioner...