Hi
Welcome to Rootschat
Because of the size of populations in London parish church areas it isn't really possible to search London parish registers for a requested look up - searching year after year in the hope of finding an entry. This would be quite literally hours of work. St Dunstan Stepney parish registers for baptisms and marriages have been indexed by the IGI, in the case of marriages 1568-1864. Nothing is showing in the index.
If you don't have a marriage how do you know the mother's maiden name was Dalby? Is it based on this baptism?
DALBEE WILLIAMS
Christening: 28 MAR 1742 Saint Dunstan, Stepney, London,
Age at Christening: 1
Father: WILLIAM WILLIAMS
Mother: MARGARET
When was the first known child baptised at St Dunstan Stepney? This child?
REBECCA WILLIAMS
Christening: 15 JAN 1737 Saint Dunstan, Stepney, London
Age at Christening: 1
Father: WM WILLIAMS
Mother: MARGT
Therefore you would be looking for a marriage pre 1737 at most about 5 years pre 1737 based on the 1714 baptisms. As the marriage does not appear on the IGI you could search for it in Boyds marriage index
http://www.britishorigins.com/The parish registers for St Dunstan Stepney are held at the London Metropolitan Archives.
Regards
Valda