I had a fast look at the officers at the start of 1808 and 1809 while doing some other research and jotted down some quick notes.
It was sometimes usual that the establishment of the battlions in a regiment were different. In most cases the 2nd Battalion [and so on] had a lesser establishment probably due to, in theory, its being the home battalion and used as a depot. I will not be able to get the breakdown by battalion but can discover if the overall establishment for the two battalions increased and by how much between 1808 and 1809.
From what I could discover it appears that the number of Captains increased by three 1809 over 1808. Also that a number left [exchange or to half-pay] and these promotions went in the regiment with maybe one exchange and apparently the rest? by purchase.
The promotions recommended by Wellington went in the regiment, however given the communications of the time, two of the officers had already received promotion before the recommendation [by purchase?]:
Captain Thomas Aylmer to Major 2 September 1808
Lieutenant James Boyd to Captain 2 September 1808
Ensign James Wigton to Lieutenant
Lieutenant Jervoise Tinley [not Finlay which is either a typo in the book or I originally copied it down incorrectly?] had already received promotion in July and Ensign William Curzon had already received promotion in early August.
While a number of the new Lieutenants were Ensigns in the Regiment before, a number were appointed without being Ensigns in the army at large. This leads me to "speculate" that they may have been militia officers who recruited from their militia regiments for rank and that the 2nd Battlion had received a large increase in its establishment [perhaps in anticipation of being ordered for overseas duty]. At the end of the war, the army list has a return of militia officers now on half-pay who had volunteered into the line.
FYI at the start of 1808 the Field Officers were
Lieutenant Colonels [rank in regt. and in army]
Henry de Berniere 17 November 1799 - Brevet Colonel 1 January 1805
John Stewart 29 May 1806 - 13 December 1803
John Cameron 3 September 1807 - 28 May 1807
George Molle 2 June 1804 - 3 September 1803
Henry Craufurd 1 August 1804 - 13 November 1801
David Campbell - 2 August 1804 - same
De Berniere's rank was not progressive past Brevet Colonel and he was not promoted to Major General.
I'll keep this on file and take a harder look next time I can.