I forgot I had the PDF on this computer here you go:

"By mediation I understand not simply the fact that particular people passed on songs they had taken from other sources, in the form of manuscript or of print, but that in the very process of so doing their own assumptions, attitudes, likes and dislikes may well have significantly determined what they looked for, accepted and rejected. Not only that, but these people's access to sources of songs, the fact that they had the time, opportunity, motive and facilities for collecting, and a whole range of other material factors will have come into play. (D Harker, Fakesong, Introduction, p xiii)

Jack Campin may have taken it from Harker's earlier book