In 1258, Henry III agreed to accept a series of reforms in return for taxation needed to pay off debts that were owed to the Pope. This account of the Parliament, which was held in June 1258, is taken from a chronicle known as ‘The Book of Ancient Laws’ and attributed to Arnald FitzThedmar, by permission of City of London, London Metropolitan Archives.
In this year was held that Mad Parliament at Oxford, about the Feast of Saint Barnabas [11 June]; in which Parliament it was provided and ordained by certain Earls and Barons of England, that those bad customs should be abolished, through which the realm, in the time of this King, had been so long and so immoderately oppressed and aggrieved, and that, by this same King and others among the most powerful men in the realm. To which ordinances the King, though reluctantly, gave his assent, and made oath to that effect. And to carry out this matter, there were chosen the Lord Archbishop of Canterbury, the Lord Bishop of Worcester, Sir Roger Bigot, Marshal, [and] Earl of Norfolk, Sir Richard de Clare Earl of Gloucester, Sir Simon de Montfort Earl of Leicester, Sir Humphrey de Bohun Earl of Hereford, the Earl of Warewyk, the Earl of Albemarle, Hugh de Bigot, Peter de Saveye, Peter de Montfort, Roger de Mortimer, James de Audeleye, [and] John Maunsel.
At the same time also, the brothers of his lordship the King, on the mother’s side, namely, Sir Eymer [de Valence], Bishop Elect of Winchester, Sir William de Valence, who had married the daughter of Warin de Munchenesey, Sir Geoffrey de Liseny, and Sir Guy de Liseny, would not give their assent to such oath; but without leave withdrew from the said Parliament, and set out for the sea-coast with their arms and harness, and, if they only had had ships, would have embarked. Afterwards however, in a Parliament held at Winchester, they received leave from the Barons to depart from the realm of England, and a day was given them to be at Dover and set sail, the Sunday namely after the Feast of Saint Silas the Apostle [13 July]; but they were not allowed to take with them any of their treasures, save only as much as might suffice for their expenses. In the same manner, William de Saint Ermin and many other foreigners had leave; all of whom set sail on the Sunday before-mentioned, or on the morrow.
Be it observed, that by reason of the aforesaid provision and statute, so made by the said Parliament at Oxford, not being observed, the realm of England was beyond measure disturbed, and many thousands of men perished, as in this book is set forth hereafter. It should also be known, that in the aforesaid Parliament at Oxford, a Justiciar over the whole of England was elected by the Barons, in the person of Hugh Bygot, brother of the Marshal, and the Tower of London was delivered into his hands.
The same year, on the morrow of Saint Mary Magdalen [22 July] his lordship the King being at Westminster, there came certain of the twelve Barons before-mentioned to the Guildhall of London, namely, the Earl Marshal, Sir Simon de Montfort, John Fitz-Geoffrey, and others, bringing with them a certain Charter, to which were appended the seals of many Barons, as also the seal of his lordship the King and of his son Edward; who thereby gave their assent, and made oath, that they would hold and observe whatever the aforesaid Barons should provide for the advantage and amendment of the realm; the persons so sent putting the Mayor and Aldermen, and others of the City, to the question whether they would assent to the provision so made by them. The Mayor accordingly, and other citizens, who could not obtain leave to speak thereon with his lordship the King, at once holding conference among themselves, consented to observe the said provision, and made oath so to do, and set the common seal of the City to the charter before-mentioned, saving however unto them all their liberties and customs.
Afterwards, the Barons before-mentioned from day to day held conference, sometimes at the New Temple, sometimes elsewhere, as to reforming for the better the usages and customs of the realm. After this, on the Nones  of August, an edict was published in the City, that no one of the King’s household, nor any other person, should take anything in the City, except at the will of the vendors; saving however unto his lordship the King his rightful prisage of wine, that is to say, from every ship that owes full custom, two tuns of wine at the price of forty shillings. And further, that if any one should presume to contravene the same, and be convicted thereof, he should immediately be imprisoned. After this, no one of the King’s officers, nor yet any of their people, took anything, without soon after paying the vendor for the same: this, however, lasted for a short time only.
In this year was held the Mad Parliament at Oxford on 11 June. Here it was decided by certain earls and barons of England, that the bad laws, which had oppressed the realm during the reign of King Henry III, should be abolished. These bad laws had resulted from actions taken by the king himself, and also because of the actions taken by other powerful men in the realm. The king reluctantly agreed to these decisions, and made an oath giving his assent.
At the same time also, the king’s half-brothers, Sir Eymer de Valence, bishop of Winchester, Sir William de Valence, Sir Geoffrey de Liseny, and Sir Guy de Liseny, would not give their assent to such oath. Without permission, they departed from the parliament, and set out for the sea-coast with their weapons, and if they had had any ships, they would have set sail [departed from England].
‘The same year, on 22 July at Westminster, King Henry III and Prince Edward made an oath that they would respect whatever laws the barons should decide were needed for the improvement of the realm.