So this is the 1841;
John Flack, 40
Elizabeth Flack, 40
Ursla Flack, 15****
William Flack, 13
Eliza Flack, 10
Joseph Flack, 9
Sally Flack, 7****
James Flack, 4
Thomas Flack, 16 Mths
Hannah Flack, 3 Mths
Address, Decoy on Sedge Fen****
1851 Census
Decoy Fen (same address as 1841)****
Lakenheath, Suffolk
HO107/1793/333/13
John Flack, 52, Head, Mar. Landed Proprietor and Farmer 80 Acres, born Hilgay, Norfolk
Sarah Flack, 44, Wife, Mar, born Tuddenham, Suffolk
William Flack, 23, Son, Unm
James Flack, 15, Son
Thomas Flack, 13, Son
Hannah Flack. 11, Daug
Elisabeth Flack, 9, Daug
Children all born Lakenheath
So Elizabeth has died after Elizabeth 1842's Birth.
John has remarried to a SARAH.
FreeBMD 1850 to Sarah CROW or Sarah BRIDGMAN (maybe a Widow)
Ursula has married and Sally (Sarah) is with her as you have found in 1851 Census