+1
to SFM's response.
Main thing is not to be confrontational in your reps. Make sure you mention the public-spirited nature of the journeys that gave rise to the PCNs (2.3. & 4), also mentioning you accept a mistake was inadvertantly made, but you weren't aware of the offence until the first PCN arrived, by which time a further four had been committed. Point out that PCN 1 has been paid, as you accept your error.