Hello Billy
Well, the details you've downloaded for Mr G. V. Keane are very interesting.
The Garth Castle, was of course, as you say, one of the Union Castle Line. These were "the mail ships". They ploughed regularly between the UK and SA, and from this particular trip, Mocambique, with passengers, mail and cargo on board.
They would arrive in Cape Town, then on to Port Elizabeth, then East London, and on to Durban.
As this Mr Keane passenger gave his address as c/o South Africa House (near Trafalgar Square), WC2, London, I would assume, he was in transit, to settle in SA. He had neither a British address, nor yet, a South African one.
He may well have travelled on to Durban, on the same ship, 'the Cape', just being logged as his destination?
Right on Billy, we need now to discover more....