(a word indicative of a joyous conclusion!)

Thanks Moonshine!!! - the routine in the third of your links (removing cached AutomaticDestinations) worked, and my jump lists are now restored (or at least, they are present now and being rebuilt).
Note that this fix does NOT remove "Recents" from within the application, so clueless lunes like me still have some pointer to what they were last doing!

Thanks to all who offered help - it's much appreciated!