A novelist called
Mary Webb wrote a ludicrously popular romantic novel called
Precious Bane in, I think, the 1920s. Very forgettable, except that the hero was
Kester and the heroine was
Jancis.
Jancis must surely be a hybrid of Janet/Janice/Janis and
Frances. It has been used ocasionally since the book came out, and it is pretty! It seems likely that Jancey is either a form of
Jancis or an elaboration of it, perhaps based on
Janet +
Nancy.