Per my post at #40, I'm glad I'm not the only one who has found this frustrating, thus spoiling what is otherwise a superb construction. The puzzle is complex enough and doesn't need further (deliberate?) ambiguity in the preamble.
I eventually opted for the single letter on the basis that the hideous truth is what Fr sees when he puts the objects on. However, it's clear that others disagree and the point about the M character not being mentioned in the Wiki entry was at the root of my original query.
Going back 40 or so years to when I first started entering, the only way to have verified the answer would have been to go to the nearest library and read the story. Yet there's a fair chance the theme would have been rejected before publication as being too obscure and not sufficiently accessible.
I've just had a look on the Listener Crossword site and the 'Guidance for Setters' still states: 'The accessibility of thematic information is carefully considered. Although the editors would not rule out a puzzle that required solvers to visit a local library, they will try to ensure that the reference material is likely to be available there and that the extra effort will be rewarding. It should not be required simply to access a trivial piece of information that is not well-integrated into the puzzle.'
This badly needs updating. No reference at all to the internet or online tools, without which many contemporary puzzles couldn't be solved (and don't get me started on needing Excel to solve a mathematical!). For absolute certainty, this is a puzzle that would have benefited from a single source of truth being specified - either 'the text' or 'the relevant Wikipedia entry'.