The claim appears to originate speculatively in George Ballard, Memoirs of Several Ladies of Great Britain (Author, Oxford 1752), pp. 379-388, at p. 379.
Her epitaph (monumental inscription in High Laver church) was reputedly written by John Locke, see H.R. Fox Bourne, The Life of John Locke, 2 Vols (Harper & Brothers, New York 1876), II, pp. 306-07.
Benjamin Carter, 'The standing of Ralph Cudworth as a Philosopher', in G.A.J. Rogers, Tom Sorell and Jill Kraye (eds), Insiders and Outsiders in Seventeenth-Century Philosophy (Routledge 2010), pp. 99-111.
historyofparliamentonline.org
M. Knights, 'MASHAM, Sir Francis, 3rd Bt. (c.1646-1723), of Otes, High Laver, Essex', in D. Hayton, E. Cruickshanks and S. Handley (eds), The History of Parliament: the House of Commons 1690-1715 (Boydell & Brewer, 2002) Read here.
