Len, CGHGSM! I'm afraid you have the wrong war

. A look at the First City, Cape of Good Hope General Service Medal (CGHGSM) roll shows he got the CGHGSM with Basutoland clasp for the Basuto Gun War of 1880-1881. I have no idea if any attestation papers exist for this unit and conflict. The First City Regiment is still in existence so a Google search may reveal a contact for them and they might have something on him.
On Ancestry24 I see there is a record of a William Alexander Barnsley marrying a Dora Ball at the Grahamstown Cathedral but no date is given. You might be able to find a reference to documents on him in the various South African national, provincial or city archives and repositories. Here's the link to the searchable index:
www.national.archsrch.gov.za/sm300cv/smws/sm300dl
You would need to retrieve the record in person or hire a researcher to do it for you (I have no idea of costs. A Google search would probably reveal contact details for researchers at the various archives and repositories).
Cheers,
Mark
Taiwan