I do not tie into your GWATKIN families, as far as I know from what little research I have done thus far.
However I do have a Frederick Pitt GWATKIN, born 1897 in Abergavenny, Monmouthshire to Alfred GWATKIN and Elizabeth Ann MEREDITH. One individual I corresponded with on other children in that family shared a site that may be of help for finding more on military service of your Frederick Ambrose Stapleton GWATKIN and his son. It is the National Archives and includes a number of different wars. It is as I recall http://www.nationalarchives.gov.uk/documentsonline/search
The index gives the basic details and has several different databases (for different time periods). For a fee you can download additional information I understand.