In case you got no further, a David Mercer married a Nancy Birch in the September quarter 1936, in St. Helens, Ref. 8b, 1795. A David Mercer (ancestorsonboard) left Southampton for the Cape in 1936 as did a female E. M. Mercer, b. 1914. You need some paid credits to get full details of the passengers which I do not have, but it could be worth starting with the marriage certificte - it could be the details are not quite accurate and would give the exact date of the marriage and then you could check the exact details of the two passengers and see if they left after the marriage date and if they were travelling together. Hope this helps.