Was the birth cert you found issued at the time of Ian/James birth, implying it had been with him since birth, or was it issued later, say when your grandfather was trying to track down his origins?
It is interesting that Alexander Wilson was a waiter and Helen/Ellen was a waitress. Can you find an Alexander Wilson, waiter on 1901 or 1911 census? Alexander could have married (or remarried to Mary) after 1911, so you may not find him with Mary in 1911.
Another possibility is that your grandfather was keen to hide his illegitimacy when he married, so Alexander and Mary are the real names of his adoptive parents, but he just tagged on his own surname, which doesn't help you at all!