Thank you so much for your response, Melinda. After recently finding your grandfather's obituary, and then happening to chance upon Simon Cottle's 2005 genealogy query about Harold Cottle, I got very excited & tried to track your family down. (You might possibly have heard along the grapevine?) So, believe it or not, I have been trying just this past couple of weeks to draft a letter about all this complex & difficult subject, to send to your Dad, Rodger, whose address I now have. Would the family be happy for me to do that? I feel it is too much & too private to exchange on a public forum.
We are indeed talking about the same man. I have various marriage & birth certificates which show quite clearly the gradual name & identity transition from Harold Wyndham Cottle to Arthur Harold Russell (happily he kept the same birthdate, mother's name & father's [unusual] first name, though changed other details). Plus we recently got some NSW government archival photo/medical info about Harold Wyndham Cottle age 20 which establishes quite conclusively that he is the same person who we knew as Arthur Harold Russell. (He had a very distinctive wound from a lung operation.)
My husband, Ian (born 1950!) has the same father as your grandfather Noel (born 1916?) - technically that makes them half-brothers, and makes Ian an uncle to Rodger (even though Rodger might be a bit older???) BTW, Ian also has a 3 years younger sister. As far as we know, there were no other children (eg. with the lady he married between Valerie & Ian's Mum).
That said, we don't know a lot about Harold Cottle's early life - only snippets of stories from his childhood, and his various travels with his work; and some background on his Cottle family (which he didn't provide but which we've found out since).
Since we found out about Noel & Rupert/Norman back in 2002, we've been very concerned to find out whatever happened to them, and are sad to hear that your grandfather had such a hard time after his father disappeared from the scene.
I look forward to hearing back from you.