The publisher's copyright in the physical layout would be 25 years from 1939. OTOH Dom Gregory Murray's tunes are 70 years from his death in 1992. Is this light, I think not. I doubt however that anybody would complain about copying for use.
If you can find the hymnal on Google books or Archive.org or as a reprint, it's a good bet it's in the public domain. I have a PDF copy of the 1939 Westminster Hymnal, New and Revised that I downloaded from Archive.org.
My version from 1966 (new and revised) has the publishers as "Burns and Oates Ltd" some of their books also add a Washbourne!
N.B. Under U.K. copyright law if the publisher goes out of business the copyright reverts to the Crown. I should also note that the Crown does not know what copyrights it holds!
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