The flap is on the nearside exhaust pipe tip.
It is most likely to be the actuator and hence a cheap fix. Get under the car and apply vacuum to the actuator and check the flap closes/opens...
If wanting to replace the backbox alone I'd go with a BMW performance system.
Do you have catless downpipes? Bastuck back box sounds awesome with CL DPs!
I wouldnt replace mid pipes unless you want it LOUD...
Car: 2010 Jaguar XFR (Cat-back exhaust)
Gone but not forgotten: Highly Modified E90 335i LCI M Sport Manual (circa 480BHP/500lb ft)