Bradshaw (pp. 179-181) traces the male figure to an engraving by Charles Monnet titled Jason and Medea at the Altar of Diana from ‘Les Metamorphoses d’Ovide’ by L’Abbe Bannier, Paris, 1767-71. I found a copy of this book at the NY Public Library. It was stored off site, so I had to put in a request and return two days later to the Print Room to see the book by appointment. My heart was in my mouth as I carefully paged through, looking for the source engraving. Well, I found it...but, sadly, it bears little resemblance to the figure. The engraving did inspire a large composite Derby 'sacrifice' group, so perhaps Bradshaw determined that the individual figures were derivative. Who knows? Anyway, it does make sense that the Staffordshire figures depict Jason and Medea at the altar of Diana.
As mythical figures go, Medea was among the worst. She was Jason's wife and a witch...literally. The only good thing she seems to have done, was help Jason capture the Golden Fleece. When Jason deserted her, she murdered their children to exact revenge. As a sorceress, she did the unbelievable. There was much draining-and-replacing of blood, and Medea actually rejuvenated a ram, after chopping it into bits. I know we have Jason sacrificing the animal in our figure, but perhaps a blog reader can explain all this to me.
A later addition to this posting:
Andrew Dando has kindly supplied a photograph of a pair of figures from the Chelsea-Derby porcelain manufactory, c1780.