Harpeden was still seneschal of Saintonge on behalf of the Duke of Aquitaine in June 1372. When the flotilla of the Earl of Pembroke was blockaded in La Rochelle by a Castilian fleet, he worked through the night of 22/23 June to procure reinforcements. He managed to gather some Gascon men-at-arms from nearby garrisons and commandeered four barges. He also sent messengers to the Captal de Buch and Thomas Felton urging them to send reinforcements as well. The men of La Rochelle, however, refused to provide their ships. He reached Pembroke's ships shortly before dawn. In the ensuing battle, the English were defeated and Harpeden was captured.