Thanks for that. I'll file the idea away for future reference.

I have discovered that the child born in 1909 was, in fact, named Charles Roy Goyder Christie, but his father changed his name to Charles Roy Martin Christie the following year (I don't know why). I discovered this by applying for what I thought were 2 birth certificates. So the mystery is solved - there was only 1 child, but the name change got registered as a seperate birth.

Another little trap to watch out for!