The two regiments has thrown me, because there is a third, not linked to Waterloo or the Napoleonic to my knowledge , my other granddad served in WW1 with...the Somerset yeomanry a territorial cavalry unit. Back to the subject Ian Fletcher in Wellingtons Regiments refers to them as 2/40 ,so we have two respected authors on the subject not agreeing. It is a long time ago I read all I could lay my hands on regarding Waterloo , Wellington ,Napoleon etc, but for my own clarity I am planning another trip to the museum and to ask some questions, assuming they have someone there not using the same source material as us ;-) .

The 40th (2nd Somersets) had a second battalion for a while but it was disbanded. It was the 1st Battalion at Waterloo. You might therefore say they were the 1st/2nd/40th!

sommilmuseum.org.uk/somerset-lightinfantry/ a bit more clarity.