I looked at the baptism of Henry, date of birth, as you mentioned, was 26 Feb 1840.
He had a sister, Charlotte, baptised at the same time, and her date of birth was 27(?) Jul 1838.
Ready for a whole new version of the surname?

GRO index for their births, which gives a mother's maiden name of Horrod:
DONKISSON, HENRY HORROD
GRO Reference: 1840 M Quarter in KENSINGTON UNION Volume 03 Page 270
DONKISSON, CHARLOTTE MARIA HORROD
GRO Reference: 1838 S Quarter in KENSINGTON UNION Volume 03 Page 242
So now you need a marriage of a Mary/Maria Horrod to Robert (any 9 letter name starting with D)
