Thank you to simes and scrose for your clear explanation to me on "disjoint subsets" as to why candidates 1,5 and 7 can only occupy columns 2,3 and 6. Your clue which convinced me was your highlighting that "there are three cells (r2c2, r2c3 and r2c6) that have ONLY 1,5 and 7 as candidates between them.

You also asked if I had looked at pairs (group of 2 numbers) and if I understand them fully. I don't understand what you mean and would appreciate your explanation on this point.

Thank you again Bonsai Cec