Thanks for the lively responses guys! My preliminary plans will have two separate 'trays' that will go on either side of our Christmas tree on the floor. They'll be connected to each other by a series of looped tracks that will encircle the tree. One half will be London, with King's Cross Station (a mix of backdrop and scratch built 3D model; essentially half a 'tunnel') and Diagon Alley (3D models). The other half will be Hogsmeade Village & Station (3D models), Hogwarts Grounds (medium background 3D models) with Whomping Willow, Black Lake, Hagrid's Cottage, and the Quidditch Pitch and then Hogwarts Castle, again a mix of 3D model and backdrop, sitting on a cliff on top of a tunnel.
Something like this:
