Quitting and restarting Xway restored everything – very strange, just like my copy & paste problem (message 763) when a restart cleared that problem.
This has thrown up another question on reverting to earlier versions, which I did in the hope of resolving this issue. Revert To > Browse all Versions... brings up a display similar to that of Time Machine. The latter, when restoring an earlier version gives the opportunity to replace the current version or to keep both, which is invaluable if you aren't sure the version you are restoring is the correct one. Xway doesn't appear to offer that, meaning that by choosing Restore you are burning your bridges, unless a copy is made first.
Thanks for your continuing help!