Time passes and I revisit the katsura trees. Thank you to David and Nadia for their comments about the similar sounding symptoms they’ve had. I’m none the wiser, sadly! According to Strouts & Winter frost damage from late or spring frosts can account for problems particularly on thin, vulnerable stems. That would fit, I suppose, except only one of my sapling katsuras was affected, the other, almost identical one, seems unaffected.
The only other katsura trees I know about locally are at Houghall Arboretum and Durham Botanic Gardens. I haven’t visited the botanic garden recently but the Houghall trees look fine.
But a closer examination of my own sickly katsura held a surprise. Scratching the bark in several places with my fingernail showed a bright green sapwood underneath. Next spring should be interesting.
I’ve uploaded some fairly large images (click on the thumbnails below) of the living and (apparently not so) dead trees.