Hi Alistair, I'm guessing you may be a very, very busy professional, so I'll recap for you.
William Royse (b. 1838 Sheffield UK - died 10 Aug 1892 Dunedin NZ) married Catherine McCrae on 2nd September 1869, at Holy Trinity Church in East Melbourne; William was aged 31 and Kate was 25.
They had eight children - Elsie Frances, Kate, George Herbert, Mary Adelaide, William Graham, Reginald Thornhill, Nora Bell Eaton, and Olive Lee Darwent.
So if William was your Great Grandfather then your Grandfather must be either George Herbert (who went to South Africa), Reginald Thornhill (who went to UK, then returned to Australia) or William Graham (unlikely as he is my Grandfather who stayed in NZ).
Unless of course there is a family connection we don't know about. FYI I'm in touch with Australian decendants of Reginald Thornhill. If you wish to correspond my personal email is: rennes(at)xtra.co.nz