Bravo,
Effectively, LR5 & LR6 married & gave one, false, reply.
I cancelled all LR5 files on the 3 HD ( C:,K:,J: ), all LR6 back ups prior to the new installation, & bingo as they say, LR6 found the correct catalogue on all 3 HD.
I thus have my safety valves handy.
Thanks for your perseverance.