The Doctor: How can you hide something that big in a city this small?
Rose: Hold on. Hide what?
The Doctor: The transmitter. The Consciousness is controlling every single piece of plastic, so it needs a transmitter to boost the signal.
Rose: What's it look like?
The Doctor: Like a transmitter. Round and massive! Somewhere slap bang in the middle of London. A huge, metal, circular structure. Like a dish, a like a wheel, round, radial, close to where we're standing. It must be completely invisible! (Rose looks at the London Eye behind The Doctor) What? (she nods at it, The Doctor turns around) What? (Rose nods at it again) What? What is it? (he looks again) Oh! Fantastic!