From Doon with Death: Part Two - S5.E8

Although Mr. Parsons is under the impression that he and his wife knew nobody in Kingsmarkham it turns out that Margaret had two school acquaintances in the town, Helen Missal and Fabia Quadrant, who are now both married to very wealthy men. Wexford assumes that one of these men is Doon, and was having an affair with the victim, but appearances can be deceptive