Complex sentence: Brownrigg had gone to Dover and unaware that his orders at Calais had been superseded, met Nicholson and briefed him to relieve Boulogne with the 30th Infantry Brigade and the 3rd RTR.
Split into simpler sentences: Brownrigg had gone to Dover, unaware that his orders at Calais had been superseded. He met Nicholson and briefed him to relieve Boulogne with the 30th Infantry Brigade and the 3rd RTR.