The letters you have posted show that you hired a car from Enterprise, then two PCNs were sent to them during your hire period for moving traffic offences which they then paid. They are now collecting the money from you. You need to look at their conditions of hire, because at the moment, you have no standing in the matter because under the decriminalised legislation for parking and traffic offences, the owner of the vehicle has responsibility for PCNs incl paying or appealing.
