Tony,
I don't have access to my meagre book collection at the moment - as I'm making the transition back up north after five years on London
But, in addition to the suggestions you already have, I think you might also find some useful material and pointers in this chapter in particular and the book in general:

John K. Severn, ‘The Wellesleys and Iberian Diplomacy, 1808-12’, in Norman Gash (ed.), Wellington: studies in the military and political career of the first Duke of Wellington, (Manchester University Press, 1990).
Best wishes
A