The vent is behind the centre panel at the rear of the boot well.
Four black studs to be levered up with a couple of screwdrivers and the panel can be removed.
Unfortunately it has flanges at the side and bottom which seat under the adjacent trim. But it's flexible and will come out.

The vent, about 3" x 5", fits from under the bumper through an aperture on the left side of the boot well and is held in place by two butterfly type studs.

I noticed that on one these the wings hadn't fully opened and the vent and it's seal wasn't pulled as tight to the metal surround as I thought it should be.

It doesn't seem possible to remove the vent without taking of the bumper but the two studs can be loosened and this was enough movement for me to get a bead of sealant around the metal opening.

To replace the centre panel I had to remove almost all of the studs and lifting up one side of the bottom molding found that could get the panel back into place.

The vented air passes down the 'scoop' and out to the rear through two rubber flaps to the outside under the bumper.

The two flaps will only open rearwards.

If the problem is one or both of the flaps not seating, I think this is one for my dealer and possibly a replacement unit but at the moment I'm crossing my fingers that I've sorted the problem.