Heathcote showed a precocious interest in a county seat for Lincolnshire in the by-election of 1794. He postponed his pretensions, not being quite of age: apart from his youth and inexperience, as a pamphleteer pointed out, he was not even resident in the county, meant to travel abroad and was fitter for the ‘mouse hole’ of Rutland. He also lacked the requisite reputation for independence. This was so far true that in preparing the ground for the next vacancy he solicited and obtained Pitt’s support. In July 1795 a vacancy arose for Rutland by the death of his uncle and he was prepared to switch his pretensions there: but he was disqualified as high sheriff and the leading interests made this their excuse for ignoring him. He reserved his right to offer in the indefinite future. So he stood for Lincolnshire in 1796 and secured the retirement of Thorold, one of the sitting Members in opposition, who had no wish to go to the poll. He had been prepared to purchase a seat for Gatton in case of failure and transferred it to his cousin John Heathcote.
Heathcote professed support of ‘a firm and steady government’ at his nomination and, in his address of thanks, independent support of ‘a free and vigorous constitution’.
Heathcote supported Addington, who, like Pitt, paid heed to his patronage applications. The Times and Morning Chronicle contradicted an assertion that he had voted on 4 Mar. 1803 for inquiry into the Prince of Wales’s finances. On 19 May, when he complained that he was one of a number of Members not supplied with copies of the late negotiations with Buonaparte, Lord Hawkesbury solemnly replied that he ‘certainly believed that many of the copies had been made use of for purposes different from what had been intended’. Heathcote offered steady support to ministerial military measures, 23 June, disliking the idea of ‘the levy in mass’. On 14 Apr. 1804 Addington requested his attendance against Fox’s defence motion the following week and he promised it, with an apology for his detention in the country.
Out of the House, Heathcote was at first prepared to attach himself to the Friends of Constitutional Reform and was one of Brand’s junto when the Whigs and radicals met on 30 Mar. 1811 to concert measures, but, like Brand, he withdrew from the venture. At the election of 1812, after turning down Lady Monson’s offer to support him at Lincoln, he stood for Rutland and drove a ministerialist from the field. On 30 Nov. 1812 he seconded the amendment to the address, calling for peace negotiations before the war impoverished the rich as well as the ‘middle ranks of society’. In this plea he stood alone.
Heathcote opposed the address, 1 Feb. 1816, protesting at a peace ‘hostile to public liberty’ and objecting to interference in the affairs of France at a time when there was so much distress at home. His ‘obstinacy’ or ‘folly’ in dividing a depleted House when the amendment had been abandoned by the Whig leaders damaged his reputation. He had exclaimed that ‘he would be damned to Hell if there should not be a division’. He was defeated by 90 votes to 23. He again threatened to divide the House on the size of the peace establishment, 7 Feb., but desisted. He was a supporter of Brougham’s motions against the ‘holy alliance’, 9 Feb., and one of his coadjutors in the meeting at Lambton’s house nine days later to concert a protest against interference in French internal affairs. He opposed the continuation of the property tax, 18 Mar., after presenting a local petition against it on 6 Mar.; he opposed the army estimates throughout. He was lampooned by the ministerialists in their New Whig Guide at that time because of a hint in debate that the Regent might have recourse to a new ‘third party’ government: they placed him at the head of it.
Heathcote did not sign the requisition to Tierney to lead the Whigs in 1818, though he had subscribed to a Whig evening paper the year before. His attendance was selective in the Parliament of 1818. He voted for the committee on the Bank, 2 Feb. 1819, and for adding Brougham to it, 8 Feb. After obtaining leave of absence on 18 May, he was in the minority on the budget proposals, 7 June. He voted against the address, 24 Nov. 1819, and for Althorp’s motion on the state of the country, 30 Nov.: but no other votes and no speeches are known. Fox hunting and the Turf were his abiding enthusiasms: though he never laid a bet. By the same token he never contested an election.
