Henry's anger passed as quickly as it had risen, and in the spring of 1240 the Earl was again received with honour at court. It was from this moment however that his position changed. As yet it had been that of a foreigner, confounded in the eyes of the nation at large with the Poitevins and Provençals who swarmed about the court. But in the years of retirement which followed Simon's return to England his whole attitude was reversed. There was as yet no quarrel with the king: he followed him in a campaign across the Channel, and shared in his defeat at Saintes. But he was a friend of Grosseteste and a patron of the Friars, and became at last known as a steady opponent of the misrule about him. When prelates and barons chose twelve representatives to confer with Henry in 1244 Simon stood with Earl Richard of Cornwall at the head of them. A definite plan of reform disclosed his hand. The confirmation of the Charter was to be followed by the election of Justiciar, Chancellor, Treasurer, in the Great Council. Nor was this restoration of a responsible ministry enough; a perpetual Council was to attend the king and devise further reforms. The plan broke against Henry's resistance and a Papal prohibition; but from this time the Earl took his stand in the front rank of the patriot leaders. The struggle of the following years was chiefly with the exactions of the Papacy, and Simon was one of the first to sign the protest which the Parliament in 1246 addressed to the court of Rome. He was present at the Lent Parliament of 1248, and we can hardly doubt that he shared in its bold rebuke of the king's misrule and its renewed demand for the appointment of the higher officers of state by the Council. It was probably a sense of the danger of leaving at home such a centre of all efforts after reform that brought Henry to send him in the autumn of 1248 as Seneschal of Gascony to save for the Crown the last of its provinces over sea.
Threatened by France and by Navarre without as well as by revolt within, the loss of Gascony seemed close at hand; but in a few months the stern rule of the new Seneschal had quelled every open foe within or without its bounds. To bring the province to order proved a longer and a harder task. Its nobles were like the robber-nobles of the Rhine: "they rode the country by night," wrote the Earl, "like thieves, in parties of twenty or thirty or forty," and gathered in leagues against the Seneschal, who set himself to exact their dues to the Crown and to shield merchant and husbandman from their violence. For four years Earl Simon steadily warred down these robber bands, storming castles where there was need, and bridling the wilder country with a chain of forts. Hard as the task was, his real difficulty lay at home. Henry sent neither money nor men; and the Earl had to raise both from his own resources, while the men whom he was fighting found friends in Henry's council-chamber. Again and again Simon was recalled to answer charges of tyranny and extortion made by the Gascon nobles and pressed by his enemies at home on the king. Henry's feeble and impulsive temper left him open to pressure like this; and though each absence of the Earl from the province was a signal for fresh outbreaks of disorder which only his presence repressed, the deputies of its nobles were still admitted to the council-table and commissions sent over to report on the Seneschal's administration. The strife came to a head in 1252, when the commissioners reported that stern as Simon's rule had been the case was one in which sternness was needful. The English barons supported Simon, and in the face of their verdict Henry was powerless. But the king was now wholly with his enemies; and his anger broke out in a violent altercation. The Earl offered to resign his post if the money he had spent was repaid him, and appealed to Henry's word. Henry hotly retorted that he was bound by no promise to a false traitor. Simon at once gave Henry the lie; "and but that thou bearest the name of king it had been a bad hour for thee when thou utteredst such a word!" A formal reconciliation was brought about, and the Earl once more returned to Gascony, but before winter had come he was forced to withdraw to France. The greatness of his reputation was shown in an offer which its nobles made him of the regency of their realm during the absence of King Lewis from the land. But the offer was refused; and Henry, who had himself undertaken the pacification of Gascony, was glad before the close of 1253 to recall its old ruler to do the work he had failed to do.