UPDATE
So, nearly two years later from the last post on this topic and absolutely nowhere near any answers.
I have expanded that section of the tree in recent months. Although William had a lot of brothers and sisters, it appeared not many of them have offspring.
His mother and father died in 1911 and 1912 respectively, but William is not mentioned in either probate. A few of the other siblings also seem to disappear.
Although I've had a look at the 1939 register to see if there were any William Horn's, I didn't seem to find any matches.
My next idea was that maybe William Horn was in Croydon, where his son George was born. I do not quite know where to begin with searching for this though.
I tried to find a matching death certificate, with the one that was in the newspaper in 1915, but couldn't find a match on the Western Australia death certificate site, so therefore, didn't order any.
Any help or fresh ideas of where to take this next would be greatly appreciated

Thanks
Chris