As it is a leased car, it can get complicated if you stand your ground and decide to fight the matter through to adjudication. This is because the second stage of the process is the service of a Notice to Owner to the lease company, who are the registered owner on the V5C Registration Certificate for the car. We see many cases where the lessee wants to fight the matter, but the lease company just pay-up and off-charge the penalty, and add on an "admin fee". So you need to check the lease conditions with the lease company and tell us what they are and also that you have received a PCN and wish to fight it, and ask them not to pay the NtO when it arrives. .
As for the allegation, this looks to be eminently appealable because they have failed in their duty to sign the restriction properly.
Your PCN has been served for parking on the yellow line during its restricted hours and has nothing to do with the school zig-zags which indicate a " No Stopping" restriction. But there is no sign to indicate the restrictions for the yellow linem because the location is within the Croydon North Zone CPZ. Here is one of the CPZ signs as you come south on Mitcham Road
https://maps.app.goo.gl/tp9ChL6a2VEFAnHu7The sign is at one of the entrances to the zone and defines the days and hours of the single-yellow line restriction inside the zone. You will see that the restriction is Mon-Sat 9.00am to 5.00pm. However, where there are two restrictions on a street, as is the case here on Lennard Road, the guidance regarding signs is that there should be a sign for the yellow line in addition to the sign for the zig-zags. Here there isn't so you can submit representations on that basis. I think there is another case similar to yours currently running on the forum, so have a look round.