I guess I don't understand the big picture. Are you trying to use your main (not backup) catalog? It sure doesn't sound like it. Why all this talk about opening a backup catalog?
Yes I have been trying to use my main catalogue, not the back ups - I thought the main catalogue was updated in synch with the latest back up - the reason you can fall back on a back up if all goes pear-shaped. .
I have managed to resolve the issue as follows:
I accessed a copy of main Lightroom catalogue from an external hard drive - upon launching Lightroom it gave me the option to use the last saved back up Catalogue and as the default one. This workaround didn't work from the main catalogue on my main computer - recent image imports and previous work completed were missing.
I had to delete the main catalogue on my main computer and drag the copy one from the external back up HD to rectify the situation.
I have no idea why this has occurred as all I did on my main computer was delete 10 redundant back up folders after which Lightroom failed to launch citing one of the oldest back ups as missing.
I wonder why Lightroom saved the most recent back-up (upon closing Lightroom) in a Zip folder only? - not noticed this behaviour before.
The ZIPping of the backup catalog is new to LR6. Its purpose is to prevent people from directly using the backups. To correctly use a backup, it needs to be copied/unzipped (ie, restored) into the original directory where the main catalog was.