There was no mention of Charles by name or any further information on his drawing but a look around provided me with another World War One sketch by my ancestor:
Swabbing While You Wait - another drawing by Charles Buchan, 1916
I had never seen this picture before and got very excited. I will be contacting the archives to see if I can obtain a copy of the second image or at the very least if they will credit the images to Charles.
Trentham Camp and Upper Hutt District, 1914-1918