It's currently being challenged by the US embassy- they have never paid it on the basis that it's a local tax, which as an embassy they don't have to pay, rather than a charge for services, which is what Livingstone contends it is. A few other embassies aren't paying either, but its the Yanks who really get up Livingstones nose.
They currently owe about a million, and good luck to them-after all, how can it be a 'charge for services' i.e. reduced congestion, when certain vehicles are exempt? A Prius takes up just as much road space as a Mondeo!!