Thanks notrab, a clue that was too convoluted for my few remaining brain cells.
All in all this has been a slog - and not a terribly rewarding one. The quotation took ages to find on the web, only coming up (with its following line) when I had deduced and then searched for the entire line from misprints. That provided the only experience that could be described as a PDM, swiftly followed by tedious mechanical substitutions.
And surely - despite the setting constraints imposed by finding words to fit - there's something pretty unsatisfying when the final grid still contains such a host of 'wrong' letters.
Just my personal opinion. Happy to be told I've missed something deeply rewarding.