to GUEST 02 Aug 22: That mention is in Allingham's The Beckoning Lady, near the very end. The book is available online at Google Books. Here's the passage.

"As Mr Campion came up he was just in time to see him put out a foot absently to trip up an August who was flitting past, but his voice was sad to the point of unsteadiness.

'The youth of the heart and the dew in the morning,
You wake and they've left you without any warning.'

He quoted the lyric softly. 'Oh Minnie, how tragic. How awful, Minnie'.