Well, since no one else has replied, I will give it my best shot.

There is nothing about the role as such in Regulations & General Orders for the Army 1811.

As far as selection was concerned, I would guess that the Sergeant Major was simply the most senior Sergeant in the battalion (that is what the title Sergeant Major implies), and officers appointments within the battalion were all by seniority, although it is possible that selection of the Sergeant Major could have been amongst several senior sergeants by merit. It is entirely possible that there was no consistency between battalions on such matters.

Originally, Sergeants were paid 1 shilling and 10 pence per day, whilst Sergeant Majors were paid 2 shillings and 6 pence per day. In July 1813, a new post of Colour Sergeant was created. There was to be one of these per company and his pay was to be 2 shillings and 4 pence per day. On that same date the pay of the Sergeant Major was raised to 3 shillings per day (which is three times as much as a Private soldier was paid).

In the late 19th Century the post of Company Sergeant Major was created, and the Sergeant Major was retitled as Regimental Sergeant Major (RSM), even although he was really the battalion sergeant major

The modern duties of the RSM have been described as: "The RSM is responsible to the Commanding Officer for all matters of discipline, including drill, dress and deportment, and will advise on matters of custom and ceremonial within a unit and carries out other duties as advised by the Commanding Officer". In my experience he is uniquely placed to advise the Commanding Officer on many internal matters within the battalion and would invariably be consulted on promotions of sergeants etc.

Perhaps the best illustration of the role of the Sergeant Major in battle is that in the diagram on page 170 of "The Waterloo Companion" by Mark Adkin. This shows him directly behind the Colour Party, in the centre of the line of supernumerary officers and sergeants. The 18th Century "Treatise of Military Discipline" on page 6 (of my reprinted 1762 edition) mentions these supernumerary sergeants as "The remaining sergeants are to divide their ground equally in the rear, which circumstance must be carefully attended to by the Sergeant Major."

Rod