On Monmouthshire Marriages 1725-1812 - there is a John YEATS, who married an Alice JONES at Monmouth 9 October 1781.

Do you have a copy of Benjamin's marriage (for witnesses etc)?

Google Books

A topographical Dictionary of England 1831

Under the list of Subscribers Page Ixxvii

YEATES, Mr Benjamin, Monmouth
YEATES, J Esq. Hyde Street, Bloomsbury, London
YEATES J Esq.Brighton, Sussex
YEATES Mr J, Acton Green, Worcestershire
YEATS, G D Esq M.D. Acton Green, Tunbridge Wells

Google: Benjamin Yeates Monmouth

takes you straight to the website - A topographical Dictionary of England - click on this, and you go directly to the page