(The following is a large section that I rewrote and edited extensively for the Wikipedia article on Stephen Storace.)
There is no clear explanation why the Storaces abandoned Vienna at the height of their success there. The reasons are suggested to be more personal than professional. Certainly the Emperor spoke of her with great admiration, even using her abilities as an arbitrary unit of currency - "I'd not give you a Storace for it!". Quite possibly Nancy was under pressure from Elizabeth, who was not at all happy in Vienna, and wished to return to England with both of her children in tow. Nancy left Vienna in February of 1787, along with her "entourage" of Michael Kelly, her brother, and Thomas Attwood. Buoyed-up by their success on the Viennese stage, the coach-party which left for London could not have imagined they would find themselves rejected and unwanted in there, where their names were quite forgotten after such a long absence. Stephen was remembered - if at all - as an infant prodigy violinist at Vauxhall Gardens, and found it very hard to secure paying work without the cherubic charm of youth behind him, and moreover as an unknown composer.
The following, from Stephen's opera The Doctor and the Apothecary, was composed for Nancy in the spring of 1787, shortly after their return from Vienna to London. It clearly shows a strong Mozart influence.
Part II, tomorrow!