If you lived in or around London, trust me, you would know EXACTLY when and where the congestion zone is in force, fair play to them, they suspended it over Christmas & New Year and this was well advertised on local radio and again when the charges were to start again on the 4th of Jan.
If you went anywhere for the first time you would check the cost and how to pay so you all should be doing the same when you come to London.
Before you ask I can't stand Ken Livingstone but the congetion charge is fantastic for keeping all those Merc and BMW drivers out of London who think that getting a tube or bus is beneath them, roll on the £8 a day charge.