I discovered quite by accident that the Dartford crossing is changing the payment method to a non stopping (Tag) system, similar to the London Congestion charge. If you use the crossing in either direction you must pay the toll either by phone, online or at a payment office before midnight on the day following the crossing, unless you have a prepaid Tag. The toll is also rising by 25% to £2.50p.

www.highways.gov.uk/dartford