Ok I finally got an answer from someone at Little Brown.AndyJ wrote: ↑Wed Feb 23, 2022 9:48 pm Based on the general assumption, the publisher or their heirs or successors in title would be the present day copyright holders. Fortunately in this case you have a direct successor to the original publishers in the form of Little, Brown who retain the Constable imprint.
"I hope this finds you well. Your email has been passed to me in the rights team at Little, Brown. We do now look after Constable titles but I am afraid I can find no record of this title anywhere on our system, likely due to the age of the book.
I’m really sorry I cannot be of more help with this."
So that's a dead end and I really don't know where I could ask.
I tried this https://www.dacs.org.uk/licensing-works/enquiry
But Helen Stratton doesn't seem to be in their system.
Is there any institutions in the UK where they might have something about Helen Stratton's heirs ?