Thanks again for your help. I have just gained access to the 1930 census via Ancestry. This shows Mary, George and Alison still living together in Brooklyn. I have also found that Alfred's brother, George, travelled to New York in 1907 but must not have stayed long as he got married in March 1910 back in England.
The record is not very clear, but George was a plumber and it also mentions the YMCA, so maybe they were both working there together.
I have now got a copy of the marriage cert for Alfred and Mary A Smith. It gives Alfred's address as 163 Noble St and Mary's as 466 Lexington Ave. Mary's parents are Martin and Mary Ann Bothwell(?) I have looked for them on the 1910 census, but with a surname of Smith, it's a mission impossible.