I have spent much of this last week looking through Unit War Diaries – available on Ancestry.
I had already found an entry in that of 137th Field Ambulance dated 30 Sep 1917 which stated – “On September 26th two NCOs and one private of the Field Ambulance were awarded the Military Medal for an act of bravery performed on September 20th 1917”. This was written at that Field Ambulance’s HQ at Fins.
The next piece of evidence is in the Assistant Director of Medical Services for the 40th Division’s War Diary dated 26 September 1917 “The Military Medal has been awarded to two NCOs and a man of 137 Field Ambulance for gallantry near the Advanced Dressing Station Villers Guislain on the 24th inst.”
Although there is a discrepancy in dates, this is clearly referring to the same event. I have been unable to find any more details of the actual event.