In 1808, he was promoted to major-general, and following the Spanish victory over the French at Bailén, in July, the Secretary of State for War and the Colonies, Viscount Castlereagh sent Leith at the head of a second delegation (the first, consisting of three British Army officers, led by a lieutenant colonel, had reached Gijón on 27 June 1808 to assess the state of affairs from a military viewpoint) to the Junta General of Asturias, with a view to seeing how the north of Spain could best be reinforced to prevent Napoleon sending in more troops through Irun, and isolating him in Madrid or Burgos. Leith arrived in Gijón on 30 August, and joined Sir David Baird's forces in November.