It gave all of the symptoms of a flat battery due to it not charging, as jump starting it off car leads worked fine.
So, what was it? The magnets inside the starter had come unglued and were smashed inside. It could not be turned by hand, but sufficient current would occasionally turn it. It gave a sluggish start and drew sufficient current to mean the battery struggled and of course you won't get peak charging system voltage until it is charged again. It very much duplicated a faulty charging system as there was no untoward noise from the starter
Fortunately, I was aware of the issue on the 1250 Bandits and happened to have a spare starter I had scored at a low price from a late model bike a few months ago for this very eventuality (the starters are nearly $1100 NZD). The bike is all fixed again, and I referred the issue to Suzuki NZ as a starter should not collapse like this on a 40,000 km well maintained bike. I heard back, but they were unsympathetic, which isn't surprising given the bike is a K8 model.
Here's the pic of the interior showing the magnets. Not repairable.