Working from my army service numbers database I can fill in some more detail about John Allan. It's conjecture of course, but the logic is good.
We know from the autograph entry in Nurse Oliver's album that he was in Chailey on 23rd October 1915. He drew the Gordon Highlanders' cap badge and wrote underneath it:
J T Allan
4th Gordon Hrs
Wounded at Houdge
25th Sept 1915
The National Archives comes up with one match and this is 3246 Private John Thomson Allan. Looking at my army service numbers database I see that this number would have been issued either in late November 1914 (3243 was issued on the 28th), or early December 1914 (3250 was issued on the 3rd). John Allan's number falls almost smack bang in the middle of these two numbers so it's reasonable to assume that he joined up at this time. Furthermore, the majority of enlistments into the 4th Gordon Highlanders at this time were in the reserve battalion, the 2/4th. So I'm guessing that John Allan was posted to the 2/4th and then subsequently to the 1/4th where he was later wounded at Loos. Thankfully, in terms of trying to understand army service numbers during the First World War, Territorial Force soldiers enlisting in second and third line battalions (eg 2/4th. 3/4th etc) were given numbers from the same series used by the 1/4th.