Hello Lyndsey & Michael,
Been out all day, glad you found what I dug up was interesting, and please that there was some new information for you

Michael, I thought at first Septimus meant 7th child, but until Lyndsey finds all the children in the BTs we won't know, he could be 7th son as there were two marriages. (I thought she'd like to search herself, I found several and there are more probably!)
1851 census HO107 Piece 2527 Folio 791 Page 16
Holland House, Clarendon Road, St. Helier, Jersey
John S Donaldson Selby head m 56 Barrister at Law and Justice of the Peace
Mary Donaldson Selby wife m 54
Thomas D Selby son unm 26 No Profession
Henry D Selby " " 25 Lieutenant Royal Navy
Septimus D Selby " 13 Scholar
Constantia G D Selby dau 14 Scholar at Home
Dorothy Grey Clark visitor unm 35 No Profession
all born England
Jane Noalles servant unm 23 Cook St. Brelades Jersey
Jane Barrow " " 21 Housemaid England
Barbara