I can't add anything to the search for the marriage, but it took place in 1740. I've copied this summary of a document held at the Somerset RO:
DD\HY/3/5/1
Title Deeds relating to lands in Kilmersdon, with map of coal mines.
Description In 1692 Mary Croke and John Lisle, Esq., of Dibden, Hampshire conveyed property to James Board of Kilmersdon, maltster, as follows: a messuage divided into 9 parts, with 2 closes called Polling Downs (5a), Downs Closes (6a), and 5a of arable land in the common fields of Charlton, all in Kilmersdon. Eight parts of the lands had earlier formed a fine between Francis Collard, yeoman and Lislebone Longe, Esq., and Frances his wife, 1671. The lands remained in the Board family, being settled on the marriages of John and Mary Board, 1700, and Francis Board and Susannah Millard of Street, spinster, 1740. They were mortgaged in 1800 to William Nuth of Newbury, Kilmersdon, gent., with further charges, 1800 and 1805, and conveyed to Thomas Samuel Jolliffe, Esq., in 1812.
It's possible John and Mary were the parents of Francis. If you search the Somerset archive online and enter "Board" and "Kilmersdon" there are other documents there relating to the family
Steve