Thanks to everyone for the help so far
could she have been giving the wrong father's name for any reason?
Anything is possible, agho, my grandmother being a case in point. On the marriage certificate to my grandfather she described herself as Cecil Maud ROLFE, Widow and her father's name as Edward John CASE. It's only when you find them on the 1911 census that you see she was his stepdaughter and her name is given as GASCOYNE. We were lucky she hadn't taken his name or she would have been very hard to find.
Unless we get lucky with Ellen, I'm going to have to assume she was telling the biological truth, at least until events show otherwise.
Thanks again to all for the advice so far
Tony