Philip Haythornthwaite writes that: 'Literacy was essential to promotion to sergeant, a sergeant being required to keep the company's clerical records, but to judge from some extant order-books, the definition of literacy was sometimes wide! By no means all sergeants were models of propriety...' (Armies of Wellington p 53)

Haythornthwaite's account also implies that promotion to be a corporal or a sergeant was decided within the regiment (presumably by the commanding officer of the battalion (at least usually) on the recommendation of the company's commander, although I don't know that I have seen this actually spelt out and explained. This probably meant that it was up to these officers to decide the level of literacy they required in any particular case. Private Wheeler declined an offer of promotion for fear of failure and consequent punishment, but was forced to accept it a few months later (p 109 of his Letters).

Otherwise the Ellis article is very good (at least that's what I say in my notes, I honestly can't recall reading it!).

There is not much else on the subject, although some good first hand material in some of the memoirs.