Thanks for trying to help, Barbara, much appreciated.
I have Just, yes, Just, found someone who may be their child. Augustus George H Maund. Augusts was the name of Elizabeth's uncle and George was her father, the H could be Hardy - or it could be Horace or Herbert or Henry! James does not seem to have got a look in though, strange for a first son. His father was James.
The child was born in 1882, it will not take me much further, but it will give me a descendant if he is their son. Something we did not know before and will surprise the family, that is if it is.
My husband is descended from her elder son who was not James' child. 14 year age gap.
I have consulted both Ellis Island and Castle Garden for America, though not Canada yet. I have looked at most of the Australian and New Zealand records on line.
Thank you
Pelican