The exhaust trims are NOT part of the bumper. I personally have fitted the exact same bumper as yours (BTCC/Aero kit) to my
Cupra and that rear bumper comes with 3 sets of exhaust trims to fit either the dual as you and I have, the central exhaust same as
K1 and the standard exhaust. The bumper is marked on the inside for each exhaust and you have to cut the bumper out to suit the exhaust you fit. In each case the trims are fitted with self tapping screws to the bumper from the inside.
They may not be shown on a parts diagram as I am not sure if they are available to buy seperately.