Sumsion maintained personal friendships with many of the well-known composers who frequented the festival, particularly Vaughan Williams, Finzi, and Howells. Social gatherings at the festival, in which Sumsion's wife Alice played a significant role, helped to cultivate these relationships. In 2007 Ursula Vaughan Williams still recalled that her husband had been 'great friends' with Herbert and Alice. Sumsion was considered part of Finzi's intimate circle and was a frequent guest at Finzi's home at Ashmansworth in Hampshire; the Sumsion and Finzi families (the Sumsions had three sons, the Finzis two) also went on holiday together. The Sumsion and Howells families were likewise close, evidenced by the fact that in 1935 the Sumsions hosted Howells and his wife for Christmas following the death of the Howells's son Michael; and according to one Howells biographer, it was Sumsion who first encouraged Howells to allow "Hymnus Paradisi", written in Michael's memory, to be performed.