Complex sentence: Keith, concerned for the undefended base at Minorca did not steer for Rosas immediately, instead sailing via the island to collect "Ville de Paris" which had left St Vincent at Port Mahon.
Split into simpler sentences: Keith, concerned for the undefended base at Minorca, did not steer for Rosas immediately. Instead he first sailed via the island to collect "Ville de Paris", which had left St Vincent at Port Mahon.