This considerable work was nearly lost to us when Howells’s bag was stolen from him and thrown from a train, and fortunately found by police. It was composed in 1954 but not revised until 1982 for a celebration of his 90th birthday at the Royal Festival Hall.
The ‘Sabriensis’ refers to the River Severn which flows through the Three Choirs Festival cities of Worcester (where it was first performed) and particularly Gloucester where Howells had studied as an organist.
We can do no better than read Jonathan Clinch’s impressive notes from Hyperion Records (© 2020 ):
https://www.hyperion-records.co.uk/dw.asp?dc=W21682_68294
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