Have you considered contacting the Dartford Crossing people just in case the demand is genuine and was prompted by a car using a cloned number plate? I would. For two reasons.
If it's happened once it can happen again or earn you parking fees and speeding tickets. If you have a clone plate out there, you at least want it logged with police.
If the demand is genuine and you ingore it, you could face larger demands for late/non payment or even end up in court or facing bailiffs or court sheriffs. And of course having your credit rating damaged.
It is impossible for anything digital to sound analogue, because it isn't analogue!