Grosmont was present at the great English victory in the naval Battle of Sluys in 1340. Later the same year, he was required to commit himself as hostage in the Low Countries for the king's considerable debts. He remained hostage until the next year and had to pay a large ransom for his own release. On his return he was made the king's lieutenant in the north and stayed at Roxburgh until 1342. The next years he spent in diplomatic negotiations in the Low Countries, Castile and Avignon.