There isn't much point in making a complaint. The police have no obligation to be helpful and the onus is on the recipient of the notice to make enquiries about dates, etc. The time and place to make representations is in court when a prosecution is attempted. As I pointed out earlier, if a NIP is not served on the Registered Keeper within 14 days and there is no good reason for that failure no prosecution is possible. If the fixed penalty has been paid that's that as far as the police are concerned.