There were two portraits made of Universalist minister (and John Murray's mentor) James Relly, according to the Dictionary of National Biography, but I've only ever seen one, as seen in this article about him. That's surely the one by John June, an engraver who "flourished c. 1760-1770 . . . of no great eminence".
Sylvester Harding is supposed to have made another portrait, and presumably there's an image of it somewhere but I've never seen it. Have you? Perhaps re-reproduced in a book frontispiece. Putting this out there for a long-term inquiry.