[3] In November 1678, Charles II gave the royal assent to the second Test Act which removed all Catholic peers, including Petre, from the House of Lords.
However, the paucity of evidence to substantiate the charge of high treason, and the weakening of the Whigs in 1681–82, lessened the chances of Petre being convicted, and he continued to be held without trial.
Petre resorted to many tactics to use, to the full, the lands that he retained, to cover his capital losses, and to postpone or to ease his payments to his dependants.
To which they pleaded in defence Parliament's order for raising £3,000 by the sale of his wood (perhaps the ordinance for funding Colonel Harvey's regiment of horse was never fulfilled).
Arthur Barnardiston was appointed, and shortly after Mr. Richard Greaves, of Lincoln's Inn, held the office, and was directed to keep court on Lord Petre's estate.
By this act of apostasy, Petre recovered his estates, but it is clear from his last letter, written in the Tower to Charles II shortly before his death, that he died a Catholic.
In his diary, Samuel Pepys, who was acting as agent for his relative William Joyce, a chandler who was attempting to recover a debt he was owed by Lady Petre, makes no secret of his opinion of her "a drunken jade",[7] and a "lewd woman", though he admitted that she was handsome and high-spirited.
[13] In 1678 Oates swore in his deposition before Sir Edmund Berry Godfrey that he had seen ‘Lord Petre receive a commission as lieutenant-general of the Popish Army destined for the invasion of England from the hands of Father d'Oliva (i.e. Giovanni Paolo Oliva), the General of the Jesuits.'
[15] William was arrested with four other Roman Catholic lords – Powis, Belasyse, Arundel and Stafford – who were similarly accused of being destined for high office under the Jesuitical regime, Petre was committed to the Tower on 28 October 1678.
A few months later Viscount Stafford was tried, condemned and executed;[17] but the patrons of the plot derived no benefit from his death, and nothing was said of the trial of the other ‘popish lords’, though the government took no steps to release them.
Under English law, two eyewitnesses were required to prove an act of treason, and unfair though the Plot trials generally were, the judges were scrupulous in observing this rule.
Nevertheless, Petre, who was already an old man, suffered greatly in health, and when, in the autumn of 1683, he felt that he had not long to live, he drew up a pathetic letter to the king.
Lord Petre was fortunate in so far that he did not lose his head, but he protested his innocence in vain; it was remembered against him that undoubtedly foreign Papists had frequented Ingatestone Hall.
It was useless to expect a Stuart to remember and feel any gratitude for the fact that Lord Petre had suffered and fought on the Royalist side so few years ago.
A writ of habeas corpus on 12 February 1684 was issued and the judges of the King's Bench declared that these men should long ago have been admitted to bail.
Mary, who was born in Covent Garden on 25 March 1679, married on 14 April 1696, George Heneage of Hainton in Lincolnshire, and died on 4 June 1704.
His aged mother survived him just a year, and was buried on 17 January 1685, on the north side of the then-new south chancel, and not in the vault with her husband and son.