Alan
I came across the citation, and also that for his the Bar to his MM, held within private papers at Liverpool Museum.
The first thing to note is (as Frank says), MM citations were not published in the LG.
There is occasionally reference to them in the War Diary (my grandfather's weren't), but in my 15 years of research I have always found that the best place to find the reason for an MM award is the local newspaper, as family would often go along and provide details. This would then be published, often with a picture of the recipient.
Here is an example of a War Diary entry, which reads "Notification was received that Sgts Gulliver & Flanders(?) had been awarded the Military Medal." This is 11th Bn, King's (Liverpool) Regiment and is dated 3rd September 1916
No details of what for or when !!