William Allen married for the third time in 1827. Grizell was the eldest sister of another family of well-off Stoke Newington Quakers, of whom the best-known is Samuel Hoare Jr (1751–1825), one of the twelve founding members of the Society for the Abolition of the Slave Trade. She had been nurse and companion to her father, a merchant in the City of London, and then married Wilson Birkbeck in 1801. As a wealthy widow, she contributed to the 1824 foundation of Newington Academy for Girls, and three years later she and William Allen, both co-founders of this novel educational establishment, married. She was 72, and the marriage was greeted by a satirical cartoon entitled "Sweet William & Grizzell-or- Newington nunnery in an uproar!!!" by Robert Cruikshank.