Or maybe Troy is a suspect but Harriet isn't - perhaps the murder victim is, say, a vicious art critic who recently wrote a terrible review of Troy's work. Or something. I just don't think I can bear to put Harriet through suspect-hood again.

Although on the whole I think it's better if neither of them is a suspect, and they get drawn into the case because one or both of them has a personal connection to the victim. It gives the story a different kind of emotional resonance that way. And as long as Harriet and Troy aren't suspects, Peter and Alleyn won't rush back to get involved.