Hi Tess
When you wrote, what name did you say you were looking for?
The posts here suggest his birth was registered in the surname of Sage as he was born before his parents married.
You can apply to the GRO online at
www.gro.gov.uk, when you apply you should fill in the online form stating you know the GRO reference and the year.
You should enter the surname as Sage and the first names as George King. The rest of the reference is this
April/May/Jun 1857
Hollingborn
Ref 2a 432
Hope this helps
Dawn