Ann Sago and the family are in London in 1871, still no John:
RG 10/580 Folio 64 page 42
Poplar All Saints
29 Little James Street
George Sago, 38, labourer, b. Yarmouth Norfolk
Ann, wife, 23, b. Ireland Cork
Amelia, 6, b. Poplar Middlesex
Augustus, 3, b. Poplar Middlesex
Given George's age, is it possible John was from a first marriage of George? (or illegitimate?), and that's why he's being difficult to find?