Having trouble with this one

William Strang c1845 possibly illegitimate to Margaret Henderson, first sighted 1851 New Grahamstown Port of Monteith with mother Margaret and half brother James McKay (my g/g/grandfather) also grandfather James Henderson c1776 Port of Monteith.
1881 with a Janet Strang (wife ??) and a daughter Christina c1876 Stirling.
Nothing for him after 1881.
Janet and Christina in 1891 in Stirling and in 1901 at Glasgow Kelvinside..that's if I am on the right track

Problem is I can find no evidence of birth, his marriage or his death.
Anyone have any other ideas please... Nothing on SP ??
Regards patvp