Without seeing the NtK, it is difficult to advise properly. If the station is owned by TfL or any other train operator, then it is likely to be land under statutory control, i.e. bylaws.
As it’s Saba that have issued it, it could be either a Penalty Notice (PN) or a Parking Charge Notice (PCN). Either way, under no circumstances should the drivers identity be revealed. All communication/appeals should be as the keeper.
As it is likely to be an invoice to the known keeper informing them that the unknown driver is liable, the fact is that they cannot rely on the provisions of PoFA to transfer the unknown drivers liability to the known keeper.
Because the unknown driver is always liable, the only way they can be identified is if the known keeper tells Saba, inadvertently or otherwise.
As the known keeper is under no legal obligation to identify the unknown driver, Saba are in a Catch 22 situation. They are hoping that the known keeper is low-hanging fruit on the gullible tree and will simply pay the unknown drivers charge or reveal the drivers identity, thus changing the liable party from unknown to known.
The next question is whether the NtK is a PN or a PCN. If the former, it will time out after 6 months. If the latter, Saba can try and take it all the way to court for up to 6 years after the parking event. More importantly, only the unknown driver can be liable unless the known keeper has identified themself to also having been the driver. Hence the advice not to identify the driver.
OP, do you see where we are going with this? Whatever mitigation you put forward, solely as the known keeper, does not matter as long as Saba cannot transfer liability to you, which they can’t, because they cannot rely on the provisions of PoFA to do so.
