Pay on the day (online available). If you went into the zone and didn't pay you now owe at least £50. Ca-ching £££.
What happens if I forget to pay?
At midnight, images of all of the vehicles that have been in the congestion charging zone are checked against the vehicle registration numbers of vehicles which have paid their congestion charge for that day. The computer keeps the registration numbers of vehicles that should have paid but not done so. A Penalty Charge Notice will be issued to the registered keeper of the vehicle.
AND
Following a final check at midnight, the computer will keep the registration numbers of vehicles that should have paid but not done so. We will then manually check each recorded image and issue a Penalty Charge Notice of £100 to the registered keeper or hirer of all those vehicles. As with parking penalties, this will be reduced to £50 for prompt payment within 14 days. Failure to pay the penalty charge within 28 days will result in the penalty being increased to £150.
http://www.cclondon.com/paymentsandpenalties.shtml
