 His captivity caused St John to be in heavy debts and was forced to pledge four of his manors for sixteen years to the merchants of the society of the Buonsignori of Siena.On 3 January 1300, St John was appointed the king's lieutenant and captain in Cumberland, Westmoreland, Lancashire, Annandale, and the other marches west of Roxburgh.During the famous siege of Carlaverock in 1300, St John was entrusted with the custody of Prince Edward of Carnarvon, Edward I's son, who was taking part in his first campaign.