I have now found SG Applin's CG Service Record (ADM 175/82A pdf 76-77) this can be downloaded for free from the National Archives, but be warned it is a very large file. You will see from this that he was already serving in the CG at the outbreak of WW1.
It does contain some family information, such as the number of children, their sex and dates of birth, but no names. It also gives his wifes place of birth, but no other information.
Martin