I found it! The story of the interrupted Court Martial at Corunna on 16th January 1809 is in the reminiscences / diary of Thomas Dundas of Fingask, (pp 212-215), then an Ensign in the 1st Battalion 52nd Foot:

After retreating through Galicia without the benefit of decent food, the young officers were roasting a bullocks heart when at mid-day officers of 52nd Foot and at least 2 cavalry officers were required for the board of a court martial. Having delivered their verdict they wait for their Captain to return before eating, only for him to tell them a 'quarter past three' that the Court Martial verdict has been 'reversed' and that they must 'reconsider the sentence'. Again the Captain has to take the sentence to the Colonel, and while he is gone, and the officers are waiting to eat, the French artillery open the battle - Thomas Dundas is posted as Ensign to the Regimental Colour.

This, however, does not appear to be the Court Martial of the Sergeant Major of the 92nd. I have conflated the two incidents.

Thank you again to all those who tried to help me find the elusive entry last year. GD