According to the private journal of Elder Peleg Burroughs of Tiverton, a Minister of the Gospel, "on the 8th day of September A. D. 1780 -- Preached at B. Hambly's upon Psalm 107:31 (O that men would praise xc) married John Manchester to M.F. and stayed all night at Bro. Davenports."
Benjamin Hambly's wife was Elizabeth Fish, probably related to Mary (needs more research).
In the 1850 Federal Census, William is age 63 and living at Portsmouth, RI, near his brother John. With him is son William M. Manchester, age 36, and also Abby B. Bennett, 29. Abby may be his niece, through either the Bennett or the Fish family
From notes of Glenn Russell at Rootsweb's Worldconnect: William Manchester's will dated 21 Nov 1883 and probated 12 Jan 1885 at Portsmouth, Newport Co., RI mentions son, William M.; John, son of late brother, Edward; Ann Augusta, Clara W. Rathbone, and Bertha L. -- all daughters of John; unnamed daughters of Henry King of Tiverton, granddaughters of brother John.