This popular melody is called "Cwm Rhondda" meaning the Rhondda Valley. The hymn "Arglwydd, arwain trwy’r anialwch" was written by William Williams in the 18th Century (with English versions called "Guide Me, O Thou Great Redeemer" or "Guide Me, O Thou Great Jehovah"), and previously sung to alternative tunes. It wasn't until much later that another Welshman John Hughes (1873-1932) created this more memorable melody to which the hymn is now sung. Because of its repeated refrain in the first verse the tune is sometimes called "Bread of Heaven" or "Bread from Heaven" and it has also been adopted as a Welsh Rugby Song. The Welsh nation is noted for its singing and here comedian Michael McIntyre demonstrates how a Welsh audience will (with very little prompting) sing Bread from Heaven in harmony! The hymn was also used by composer Alfred Newman in his oscar-winning score for the film "How Green Was My Valley" set in a Welsh mining town and directed by John Ford.
Here is a video of Cwm Rhondda for piano:
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The image below is the first page of the sheet music, and here you can download the full sheet music for Cwm Rhondda for piano in PDF format.