As Seymour has his head out the truck window, looking back and talking to Compo and Cleggy, it's very obvious that he reaches in to open the door, trying to make it look to the audience like it opens accidentally as the truck goes around a corner.
When Compo is luring Nora away from her house so that Wally can sneak out, Compo goes around the corner of her house, sneaks a look back at Nora to make sure she is following, and and goes across the lane and around another building. Nora does follow, and, as she turns around the other building, Compo comes back into view and rushes to get Wally. However, after Compo has looked back at Nora and disappears from view, a double for Bill Owen takes over and goes to the other building. The use of the double is obvious for two reasons. 1 - After Compo goes out of sight the first time, the double takes off too quickly. 2 - Compo/Bill Owen comes back into view too soon. He couldn't have gotten around that other building so fast.