Grenfell had prospered as the associate and partner in the copper trade of the Williams family, to whom he owed the seat for Great Marlow which he had occupied as a Grenvillite Whig and loquacious financial pundit since 1802. His partnership with Owen Williams, the other Member for Marlow, in the London copper agency at Castle Baynard wharf, Upper Thames Street continued, and he operated profitable smelting concerns in the Swansea area of South Wales.
He continued to attend regularly and acted with the Whig opposition on many issues, although his record on economy and retrenchment motions was mixed and he remained lukewarm about parliamentary reform. He divided for Catholic relief, 28 Feb. 1821, 1 Mar., 21 Apr., 10 May 1825. On 28 Apr. 1820 he secured a return of bank notes in circulation, as he did in successive sessions of the 1820 Parliament.
Grenfell was in the opposition minority on the address, 5 Feb., yet voted with ministers against more extensive tax reductions, 11, 21 Feb. 1822. He gave ‘hearty support’ to admiralty reductions, 1 Mar., and called for repeal of the ‘most objectionable’ salt tax, 1 Apr., 30 May, 2 June.
On 15 Feb. 1825 Grenfell expressed support for the bill to suppress the Catholic Association, which ‘impeded the pure stream of justice’, but he raised eyebrows with his ‘fervent prayer’ for successful Irish Catholic resistance to their being ‘oppressed, injured, insulted [and] trampled upon’ by the Protestant majority. Six days later he recanted and opposed the measure. He carried the second reading of his reintroduced St. Katharine’s Docks bill by 119-30, 22 Feb., though his own vote was subsequently disallowed after he admitted being a subscriber to the scheme. He said he wanted Members with vested interests in opposing private bills to be prevented from voting on them, 23 Feb., 10 Mar. He steered the docks bill through the Commons and it gained royal assent, 10 June.
In 1829 Grenfell fell out with Owen Williams, who vowed ‘never [to] make it up with’ him, over the conduct of the copper business, apparently on account of the large sums which the ‘overbearing’ Grenfell had borrowed without consulting him. Williams had by then withdrawn from the enterprise, which continued as Pascoe Grenfell and Company.
