Cust, who had been returned for Grantham on his brother Lord Brownlow’s interest in 1818, offered again in 1820. He defended his parliamentary record and declared that the country owed its good sense and its character to the industry of the middling classes. Emphasizing his support for the constitution in church and state, he urged the electors to ‘exert themselves ... to prevent the propagation of doctrines which, if successful, would strike at the root of all we hold dear and valuable’, and cautioned the lower classes against being duped by the promises of radicals, which could only lead to the rise of another Buonaparte. On being returned at the head of the poll he said that he yielded to no one in his determination to act independently, ‘in what appears to me to be the path of duty and honour’.
He was a fairly regular attender who gave general support to Lord Liverpool’s ministry, although there is some difficulty in distinguishing his votes from those of his brothers. He divided against economies in revenue collection, 4 July 1820. He voted in defence of ministers’ conduct towards Queen Caroline, 6 Feb. 1821. He divided against Catholic relief, 28 Feb. He voted against repeal of the additional malt duty, 3 Apr., and disfranchisement of civil officers of the ordnance, 12 Apr. 1821. He divided against more extensive tax reductions, 11, 21 Feb., abolition of one of the joint-postmasterships, 13 Mar., and repeal of the salt duties, 28 June 1822. He voted against relieving Catholic peers of their disabilities, 30 Apr., and inquiries into Irish tithes, 19 June, and the lord advocate’s conduct towards the Scottish press, 25 June 1822. He divided against parliamentary reform, 20 Feb., and reform in Scotland, 2 June 1823. He voted against reduction of the assessed taxes, 10 Mar., repeal of the Foreign Enlistment Act, 16 Apr., and inquiry into the prosecution of the Dublin Orange rioters, 22 Apr. 1823. As a plantation owner in the West Indies, he was appointed that session to the standing committee of the West India planters and merchants.
He was not without a seat for long, being returned for Lostwithiel as a paying guest of Lord Mount Edgcumbe on a vacancy in December 1826. He divided against Catholic relief, 6 Mar. 1827. In January 1828, following the formation of the duke of Wellington’s ministry, Cust wrote to the leader of the Commons, Peel, expressing a desire to be ‘actively useful’ and offering to move the address; this was not taken up.
The ministry regarded Cust as one of their ‘friends’, and he duly voted with them in the crucial civil list division, 15 Nov. 1830. He called for the establishment of some authority to control the ‘lavish and unlimited expenditure’ on public buildings such as Buckingham House and the new law courts, 15 Feb. 1831. He divided against the second reading of the Grey ministry’s reform bill, 22 Mar., and for Gascoyne’s wrecking amendment, 19 Apr. 1831. He was again returned unopposed for Lostwithiel at the ensuing general election. He divided against the second reading of the reintroduced reform bill, 6 July, and for Gordon’s adjournment motion, 12 July. He voted to use the 1831 census for the purpose of determining the disfranchisement schedules, 19 July, and to postpone consideration of Chippenham’s inclusion in schedule B, 27 July. He divided against the bill’s passage, 21 Sept., and the second reading of the Scottish bill, 23 Sept. He voted to abolish the Maynooth grant, 26 Sept. Following the report of the select committee (of which he was member) on the House’s accommodation, 11 Oct., he said it was obvious that the office provision was ‘extremely inconvenient’ and he hoped that ‘effectual steps’ would be taken. He divided against the second reading of the revised reform bill, 17 Dec. 1831, having expressed disappointment that Lostwithiel still faced complete disfranchisement. He believed the bill was fundamentally ‘the same in principle’ as its predecessors and would still make ‘dangerous inroads upon the constitution’. He welcomed the reprieve given to resident freemen voters, but warned that the disfranchisement of small boroughs would ‘deprive the colonists of making their wants and wishes known to the legislature’. He claimed that he would have been willing to support ‘a mild measure of reform’, but asserted that ministers had ‘endeavoured to catch the popular breath on every occasion, by yielding anything to the demands of the leaders of the populace’, and were apparently ‘either ignorant of, or indifferent to the consequences which may flow from their acts’. He protested in vain at the disfranchisement of Lostwithiel when boroughs with smaller populations, such as Malmesbury and Midhurst, were spared, 21 Feb. 1832. He voted against the enfranchisement of Tower Hamlets, 28 Feb., and the bill’s third reading, 22 Mar. He divided against ministers on the Russian-Dutch loan, 26 Jan., 12 July 1832.
Cust did not seek another seat after the dissolution in 1832. He visited his plantation in 1838 and on his return published Reflections on West Indian Affairs (1839), in which he stated that in hindsight he regretted that the process of abolishing slavery had not commenced immediately after the parliamentary resolutions of 1823. He also wrote The Colonies and Colonial Government (1845). He became a respected military historian, producing a five-volume Annals of the Wars of the Eighteenth Century (1857), a four-volume companion on the nineteenth century (1862), and a three-volume study, Lives of the Warriors of France and England (1865-9). He also edited two volumes of religious writings, Noctes Dominicae (1848) and Family Reading (1850), based on the New Testament. He died in January 1878 and was succeeded by his only son Leopold, who died suddenly seven weeks later.
