Thank you, yes that is indeed the correct memorial stone (I must have misread the date when looking at the stone in person due to the writing being difficult to search).
The sites I have used (from memory) over the years include: Ancestry, FindmyPast, Family Search, Essex Archives, FreeReg. Unfortunately I don't have any birth years for any of the siblings.
I've found many baptism records for Livermores with the sibling names, but none so far where more than one sibling is listed in the same place -- so it's been like searching for a needle in a haystack. Locations connected to the family from will mentions include Dunmow, High/Good Easter and Wethersfield.