So, here is an interesting turn of events. I received Mary Ann Doubtfire's birth certificate.
I was expecting it to say her mother was Sarah Doubtfire formally Earl, as her birth in 1857 was nine years before the record I found Richard Doubtfire and Sarah Tyler's marriage in 1866.
However, Mary's birth certificate says she was born 20th October 1857 at 15 Mead Street to Richard Doubtfire and Sarah Doubtfire formally Tyler!
So now where do I go? This Mary isn't the full sister of my ancestor Richard Doubtfire b.1849.
I now have no lead on his family as the one I was convinced was his turns out not to be, as far as I can tell.
It's the wrong Sarah.
(Wanders off to head butt a wall)