Here's some more conjecture regarding Chailey men and army service numbers and enlistment dates.
I have assumed from previous research, that the W Blanchard mentioned in Chailey Parish Magazine, is William Hugh Blanchard. The National Archives holds a medal index card for this man giving his number as G/24460 with the Royal Sussex Regiment. Chailey Parish Magazine states that W Blanchard was first with a Training Reserve Battalion in April 1917, and later with the Royal Sussex Regiment (first the Reserve Battalion and later the 9th).
The number G/24460 would have been issued to a Royal Sussex Regiment soldier probably in mid to late November 1917 and so this certainly ties in with the information I already had.