Ellenshaw's brief Wikipedia entry is here. The Ellenshaw family website is here (Peter wasn't the only Ellenshaw matte artist). A good source for information about matte art is "NZ Pete," whose site is here; I grabbed several of the images below from Pete.
I don't know if this painting was done for Ellenshaw's personal enjoyment or whether it was related to a movie project. In either case, it shows him as a landscape artist.
Besides mattes, Ellenshaw was sometimes involved with concept art for film projects.
"Mary Poppins" was a major project for Ellenshaw. Being an Englishman, creating a London panorama was probably easier than it might have been for an American.
Live action was located where the crowds are seen; the rest was largely or entirely painted.
The black area at the bottom of the upper detail image is part of the area reserved for live action. Note how freely Ellenshaw painted some of the indidental buildings.
Disney called upon Ellenshaw to paint clusters of sailing ship masts on more than one occasion.