My first post went awry a bit, I mis-read the older one's name.
William Mitchell - does the address Janitors House University Edinburgh in 1918 compute at all?
That is a C/Sgt William Mitchell whose service record does not survive but who was discharged to civilian employment in 1918. Just to note there were quite a number of William Mitchells in the Black Watch!
Duncan Mitchell - a birth date of 1882 makes it unlikely that D Mitchell in the Boer war is the same man as the D Mitchell in the Great War. However, the 1911 DM that Sinann IDs is certainly a starter for Duncan. Again no service record apparent but significant is the Distinguished Conduct Medal which the family would surely be aware of?
MaxD