This is evening prayer from the Divine Office with a text by the Benedictine Nuns of Stanbrook Abbey. It survived into CWB II with a different setting, but here it is set to O LIGHT SERENE by Howard Hughes.
The text is here.
Breviary Hymns has some background, but I can find no source other than CWB for this tune.
None of the clips on YouTube seem to be this setting.