Thanks for that, Oonagh.
There was a George Joy present in the house on the night of the murder who was Harriet's grandson according to the book I've got, it doesn't state the relationship to Thomas Joy, so I'm wondering if the Joys were in-laws of hers. My GG Grandfather's wife's maiden name was McCormack, though she was orphaned as a baby, so I can't entirely rule that out, but it's difficult to work out exactly how Harriet and Thomas were related.
Maybe a daughter's husband if it's not my GG Grandfather, or a daughter's husband's brother or something. I have just found a Thomas Joy marrying a Jessie Turner in Leeds in 1877, which would seem to rule my GG Grandfather out of the story.
All speculation so far though.