Isabella reopened negotiations in Paris, resulting in a peace treaty under which the bulk of Gascony, minus the Agenais, would be returned to England in exchange for a 50,000 mark penalty. The treaty was not popular in England because of the Agenais clause. Henry of Lancaster was amongst the first to break with Isabella and Mortimer. By 1327 Lancaster was irritated by Mortimer's behaviour and Isabella responded by beginning to sideline him from her government. Lancaster was furious over the passing of the Treaty of Northampton, and refused to attend court, mobilising support amongst the commoners of London. Isabella responded to the problems by undertaking a wide reform of royal administration, local law enforcement. In a move guaranteed to appeal to domestic opinion, Isabella also decided to pursue Edward III's claim on the French throne, sending her advisers to France to demand official recognition of his claim. The French nobility were unimpressed and, since Isabella lacked the funds to begin any military campaign, she began to court the opinion of France's neighbours, including proposing the marriage of her son John to the Castilian royal family. By the end of 1328 the situation had descended into near civil war once again, with Lancaster mobilising his army against Isabella and Mortimer. In January 1329 Isabella's forces under Mortimer's command took Lancaster's stronghold of Leicester, followed by Bedford; Isabella – wearing armour, and mounted on a warhorse – and Edward III marched rapidly north, resulting in Lancaster's surrender. He escaped death but was subjected to a colossal fine, effectively crippling his power. Isabella was merciful to those who had aligned themselves with him, although some – such as her old supporter Henry de Beaumont, whose family had split from Isabella over the peace with Scotland, which had lost them huge land holdings in Scotland – fled to France. Despite Lancaster's defeat, however, discontent continued to grow. Edmund of Kent had sided with Isabella in 1326, but had since begun to question his decision and was edging back towards Edward II, his half-brother. Edmund of Kent was in conversations with other senior nobles questioning Isabella's rule, including Henry de Beaumont and Isabella de Vesci. Edmund was finally involved in a conspiracy in 1330, allegedly to restore Edward II, who, he claimed, was still alive: Isabella and Mortimer broke up the conspiracy, arresting Edmund and other supporters – including Simon Mepeham, Archbishop of Canterbury.