Thanks for the replies and the help/offers of help.
This is what I have so far.
My g grandfather, William Charles Howe Platt
From birth certificate (Q2 1854, Liverpool 8b 132)
Born 10 June 1864. Son of Charles Howe, master mariner, and Sarah Howe, formerly Bird, 51 Kay Street, Liverpool.
1871 census at 33 Beloe Street, Toxteth, Liverpool
Listed as Charles Hawe, age 6, nurse child
Living with Elizabeth and John Platt
1881 census at 33 Beloe Street, Toxteth, Liverpool
Listed as Charles H Platt, 16, adopted son
Living with Elizabeth and John Platt
In the 1861 census, his mother and father appear to be listed as living with Sarah’s mother at:
11 Court Porter Street, St Martin, Liverpool
Eliza Bird head widow 47 Ireland
Ann daur widow 20[8] Ireland
James son un 16 labourer Liverpool
Sarah How daur mar 20 Liverpool
Charles son in law mar 29 sailor Rochester
Sarah Gross granddau 16 months Liverpool
RG9; Piece: 2656; Folio: 84; Page: 31
From marriage certificate of Sarah Bird and Charles Howe (Q3 1857, Liverpool 8b 2):
Married 15 Aug 1857, parish church of St Alban
Charles Howe, 28, bachelor, mariner, Eldon Place, father William Howe, labourer
Sarah Bird, 20, spinster, no profession given, Eldon Place, father William Bird, clerk in a merchant’s office
Witnesses: Henry Johnston(?), Anne Touzel’s mark
Earlier in 1857, marriage between Anne Bird and John Touzel (Q1 Liverpool 8b 4):
Married 15 March 1857, parish church of St Alban
John Touzel, 23, bachelor, mariner, Eldon Place, father Paul Touzel, labourer
Anne Bird, 20, spinster, no profession given, Eldon Place, father William Bird, clerk
Witnesses: James Sutcliffe, Elizabeth Sutcliffe’s mark
I have a likely line of descent to living descendants for James Bird, who appears in the 1861 census, but no trace of Sarah and Charles after the birth of my g grandfather. I have also been unable to find any trace of Anne and John Touzel after their marriage in 1857.
I was trying the Channel Islands connection (because of the Touzel name and the unexplained Le Gross grandchild in the 1861 census) on the off chance that there may be some lead that would help me in finding out what happened to Sarah and Charles. At the moment, the most likely thing seems to be that they emigrated, perhaps to the same place as Anne and John Touzel and possibly with Sarah and Anne’s mother, leaving my g grandfather behind. But of course that is pure speculation at the moment.