The WATCH file, a database of Writers, Artists, and Their Copyright Holders, is an excellent resource for tracking down contact details if the rights for a title have reverted to the author or the author’s estate.
The Society of Authors Members Database is a public database of authors and translators who are members of the SoA, that can be searched by name, keyword or subject area. While not all members are included, many have public contact details for press or enquiries.
If you cannot find the title you are looking for within PLSclear, the Intellectual Property Office has published some guidance on tracking down the rightsholder of hard to find titles and the due diligence needed to apply for an Orphan Works licence.
If you are having trouble tracking down a poem or are finding it difficult to locate a poet’s contact details, The Poetry Library at the Southbank Centre has a contact page dedicated to answering questions about poetry.