On a recent night in London, behind King's Cross railway station, an ordinary-looking van has an extraordinary-looking pole sticking out of its roof. The 20-metre mobile mast is, say its creators, the future of outdoor billboard advertising; a new, disruptive technology in a field that - LED video screens notwithstanding - hasn't strayed too far beyond basic precepts of paper plastered to wood. Known as the Echo, the mast was created by Lightvert, a UK start-up that wants to take advertising to places it hasn't been before.