Sorry.. I clicked on the sovereign, then clicked on the 'open spellbook' button in the menu tab. It brought up a list of all the spells available for the spellbooks I chose, I then selected a spell I could normally cast on the overworld, to highlight the 'cast spell' button, and then selected another spell, which I had neither researched, nor could normally cast on the strategic map, and could cast the spell. Amusingly enough the range was still 'infinite' and I could hurl fireballs at enemies across the entire map.