After Bagley has lunch with his wife, she drops him back at the advertising firm's office building, but it is a different building to the one used for the interior scenes, which is the tall red building several hundred yards up the street (visible in the crane shot of their car pulling up), right next to the Lambeth bridge, as we can see from the window view in the scenes in Bagley's and Bristol's offices.
Dennis' boil grows on his right side, and when it gets bigger and finally takes over its still on the right side while his "real" head gets smaller and is on the left side. OK so far. But then they keep switching sides left to right, right to left. This happens several times.