The RSM (Regimental Sargeant Major) is an appointment held by warrant officers class 1 (WO1) in the British Army, he would have been addressed as Sir.... similar to Warrant Officers in the RAF....still classed as a SNCO and would be addressed as Sir. (in the RAF we addressed Chief Technician's as Chief and Flight Sargeant's as Flight)
During the dance Nigel Davenport (Sgt) addressed Nigel Patrick (Sgt Major) as 'Sir'. As an NCO he would not be addressed as 'Sir'.