My view is that the OP should take them all the way to London Tribunals if necessary, but we don't yet know the ownership of the car in relation to the next stage, the Notice to Owner, which is sent by post.
At the moment, the OP has submitted an informal challenge, and received the usual Fob-Off letter sent out for almost all informal challenges. Most people never take matters further and pay the discount, thus reinforcing councils in their essentially unlawful practice, like here, where the car was lawfully parked on the dropped kerb.
So if the owner can tell us whether he owns the car, and the V5C is up-to-date matters can proceed.
