I seem to be stuck on 5.18.8 of Plasma - even though I've updated apt - do I need to manually add a different source?
I've run sudo apt-get update, and looked in "Discover" and software updates on Ubuntu.
I also ran "pkcon refresh", then "pkcon update" per @chetzz 's idea below. The former seemed to do a lot of work, but the latter, said no work to do.
No errors anywhere.