From the Oxford book of ballads.
The score has the following note: 'The accompanist should play only the large notes if the vocal parts are sung by soloists, but he should play in addition as many of the notes in small type as he can manage if the work is sung by a small chorus. If, however, the chorus is a large one, the accompaniment should be played as a duet, including all the notes large or small'.
Richard Wood was the brother of Anne Wood, a singer who worked several times with Britten. He was interned in Oflag VIIb POW camp during World War II.