Had a clear out of unused software and rootfs, using dpkg. Also, removed other residual bits and it seemed to do the trick. This includes some items which I'd uninstalled ages ago but whose imprints were still cluttering the device.
<sudo reboot> now works again!

I only removed the user-software objects in the list generated by <dpkg -l|grep ^rc>, not the library objects. Does anyone know whether these are all unused as well, and if removed will allow remaining programmes to run unaltered?

Incidentally, I encountered bug 5448, which really shouldn't be 'new'. Does this mean anything or is the ouput produced in error? Can I do anything to solve it?