Definitely 'thou hast'. Oddly, I typed in the words without any title (or so I thought); I can't think where the 'hast thou' came from. If it was me, despite my conflicting recollection of launching straight into the poem, then my apologies for a stupid typo.

The British Library catalogue lists the unison song, which was published in 1949, as composed by A. Mc Leish Smith.