Claire-

There is consistent and excellent advice in this thread.

I've dealt with similar questions with regard to recording songs adapted from the poems of C. Fox Smith, and some of her poems were published after the British copyright cut-off of 1923.

Somewhere in your notes to the lyrics, printed with your CD and/or on your website, should be a credit to the composer and a source for the published poem. Contacting heirs is optional but can be a rewarding experience.

I applaud you for raising the question. Many other musicians appear indifferent to the source of their lyrics, even when the composers still walk this earth. May they endure their punishment in musician's hell for eternity (speculations will be addressed in a separate thread)!

Charley Noble