Update:
things moving slowly. SoA wrote HC saying basically that the contract says I can demand a reprint if “it is not available in any of the Publishers’ trade editions covered by this Agreement” after which, if not done, there would be reversion of rights; But that availability as Print on Demand cannot be classified as availability in a trade edition by any interpretation.
HC wrote back saying they disagreed, but they would revert the rights for one of the books. The other book they won't because they don't agree that PoD is not covered by the trade editions definition, and that other book has sold some copies.
Thereupon SoA wrote them a very sharp letter indeed and said they are passing the matter on to their legal department.
I can't imagine what inspires HC to hang on to a book that didn't earn out its advance, is not available for sale on amazon, is not available for sale on their own website, and is only available for sale via Waterstones. I can't imagine anyone searching for it on Waterstones, and anyway, what is top prevent HC buying one or two copies itself just so they can say "it sold". I don't have the royalty statement for 2013 yet but since 2012 was 0 copies sold that year I can't believe more than one or two sold. I should get the royalty statement or info form the agency next week. It's really baffling; unless they want to establish a principle.
I'm so glad the SoA is hanging tough. HC seems to be digging in their heels and it would be good to have a bit of a legal tussle. I am confident the SoA would win; at least, I hope so as it would be terrible for all authors published before PoA became an option. All a publisher would have to do is put out a PoA and voila -- no reversion of rights. This just can't be right.