Hmm, now here's a curious thing. I was going through FreeBMD and Percy's 1911 census trying to confirm who his children were, and after Arthur D (born 1910) - the last child on the census - I noticed this birth entry:
Births Jun 1912
Horne Evelyn M A Montgomery Chertsey 2a 121
Montgomery is the maiden name of Percy's first wife, known variously as Eleanor, Ellen, Annette, Annettie and Nellie Montgomery. So, obviously my first thought was, is Evelyn their last child together? Eleanor died in 1913, and Percy re-married in 1915 and was killed in 1917.
Luckily, Evelyn's
baptism record is on Ancestry, and it gives her parents as Percival William and Annette Horne. (It's transcribed as Lounette, but I think it is Annette).
I'm sure this is probably just a mistake on the baptism entry, but what do we think? My first thought was that this is very likely Percy's youngest child with his first wife. Their eldest son - a child in 1912 - was named Percival William.
EDIT: Incidentally, I'm not sure what happened to Arthur or Evelyn. There is an Arthur Dennis Horne who died in 1963, but he was born in 1900.
Ah, I think
this might be Evelyn. Just need to find the corresponding marriage.