On February 23 2009 The Prodigy will release an album called Invaders Must Die, which features a track of the same name for which a video was shot at Dungeness. The Denge sound mirrors feature from about 1 min 15 sec in.

‘Invaders Must Die’ is 40 minutes of having your head battered by future nostalgia, serotonin levels twisted by feel-good horrorcore and your synapses snapped by whiplash attitude. It’s the sound of The Prodigy mixing up genres, contorting the past and rewiring the future, ram-raiding through the tranquility of music’s status quo like a blot on the landscape of England’s dreaming.
Source: Invaders Must Die

So now we know. I just hope no morons get it into their heads to try vandalising the mirrors as depicted in the video.

The mirrors have been used by a variety of musicians, including Turin Brakes, Bass Communion and Blank & Jones.

(thanks to Ken Morrow for letting me know about the video)