I exported from the old backup collection (no files, no previews). I coloured them all red just so as to have changed metadata, then just added them all to the quick collection from the Previous Import once they were imported into the current catalog.
I've now discovered that Lightroom makes virtual copies of the previous files where there are duplicates (all were duplicates in my case), so that you don't lose any changes between what were originally in the catalog and the new imports, if anything was changed.
Bottom line, problem resolved. Lesson learned, quick collections are not to be overburdened
Thanks for your help.