There is a William Harry CUMBY born Mar qtr 1887 Wandsworth, who died Sep qtr 1887 Wandsworth. More than likely a son of Henry John and Ethel as well, which would mean they were probably together from 1886 or even 1885.
Can't see any marriage anywhere, or any sign of them, or son Harold born 1889, in 1891. Henry John is around in 1861, 1871 and 1881. If he was a mercantile clerk, he may have been out of the country.
Ian C