Am pretty sure I'm correct but Preston Derby off peak return is valid via Manchester or Crewe so by default would be ok for break of journey at Manchester Airport on return?
Friend just been told not valid and had to buy ticket from Airport back to Preston!
This is fine (sort of). There are many mapped routes that include the Wilmslow to Manchester link (which is what is required to travel via Manchester Airport), when looking at a Preston to Derby journey.
The "sort of" refers to how you get there; to stay with the mapped route you would most likely want to go Derby - Crewe; then Crewe - Manchester Airport; then Manchester Airport to Manchester Piccadilly (and on to Preston). In other words, via both Crewe and Manchester in this case. You could also go via Cheadle Hulme / Stockport, then via Wilmslow (I haven't checked if the double back between Cheadle Hulme and Stockport is permitted in this case)
As you say, you can go via Crewe or Manchester; you can also go via Tamworth or via Sheffield.
Page 1561 (!) of the Yellow Pages lists the valid map combinations:
http://iblocks-rg-publication.s3-website-eu-west-1.amazonaws.com/yellow_pages.pdf
The routeing maps enable you to visualise these:
https://data.atoc.org/routeing-maps
I can't see any relevant easements.
