so you have to take the birdcage off (two screws either end) and slip the center and birdcage off as well?
I just fit the collar to the flue and fit the flue then pull back the collar to make good. It would be a nightmare bricking round it though! If you do what you said it will pull the inner flue out with the terminal. Stupid design they should of just kept the rubber collars. The cage part of the terminal covering the air side snaps very easily too! Slowly going off Vaillants had several under warranty go wrong now!