Hi Clare
No John Sands b Heathfield in either census that I can see. However, there is a John born 1826 living in Brighthelmstone. He's not with the family in 1851, but they say the two older children still at home were born in Heathfield.....
PLUS
Although their son John was baptised in Alfriston, other children were baptised at Heathfield.
1841 : ho107. Piece 1121. Folio 35 Pg. 28
Nelson Street, Brighthelmstone
Simeon Sands 40 Labourer
Barbary 35
John 15
Barbary 9
Mary 6
Dorcas 5
All born in county
1851: HO107. Piece 1645. Folio 250. Pg.5
15 John's Place, Brighton
SANDS
Simeon Head M 46 Labourer b. Alfriston, Sussex
Barbara Wife M 45 b. Wilmonton
Mary Ann Dau S 17 b. Heathfield
Dorcas Dau S 15 b. Heathfield
Simeon Son 9 b. Brighton
Philedelphia Dau 7 b. Brighton
Elizabeth Dau 5 b. Brighton
William Walt. Son 1. b. Brighton
John
not yet found in 1851 - will keep looking
But I think this is them?
John Samuel Sands b. 23 Sept 1825 bap 18 Dec 1825 Ebenezer Independent, Alfriston Parents Simeon and Barbara
Mary Ann, Dorcas and Simeon baptised in Heathfield, the younger ones in Brighton
Ambly - saw your post after I'd typed this - glad to see marriage was for John Samuel, hadn't got that far but it ties in with the bap. record.