St Mary's Works is named after St Mary's church - commonly referred to as St Mary Coslany > Link - which adjoins it. It housed the Sexton, Son & Everard shoe factory > Link which operated from 1876 until 1976. Current plans are to convert it into 151 flats, office space, a hotel with a restaurant, galleries and shops. The building is currently being used as a space for community arts and events.
I think the path you are talking about is Marriott's Way - running from Norwich to Reepham on the course of an old railway. It's part of National Cycle Route 1 (Harwich to Hull).