Please, please tell us that you have not revealed the drivers identity when you say "So I've written them" or that you haven't sent it yet.
There is no keeper liability in Scotland (yet). Only the unknown driver is liable. The NtK was sent to the known keeper and they should have told Smart Parking that you were the known Hirer. What Smart are then supposed to do is send a new NtH addressed to you, the known Hirer.
Smart have no idea who was the driver unless you have blabbed it to them, inadvertently or otherwise. As there is no keeper liability in Scotland and they have no idea who was the driver as they are not allowed to assume or infer that it was the keeper/hirer, they would have nowhere to go with this except to try and scare you into paying into their scam with scary but harmless debt collector letters.
So, do you understand why the drivers identity should never be revealed in Scotland (for now).
If the drivers identity has not been revealed then the NtH will 99.99% likely not complied with the requirements of PoFA and so a simple appeal by the known Hirer to Smart reminding them that, as the NtH is not PoFA compliant, they should go and chase the unknown driver and good luck to them with that.
Fortunately, even if you have given them the drivers identity, they are unlikely to actually take it through Simple Procedure which is held in the Sheriff Courts in Scotland as they cannot claim any costs for a claim under £200 and the maximum costs for a claim between £200 and £1,500 is only 10%. So the maximum they could recover even if they were stupid enough to chase a £100 PCN is zero. Even if they were to inflate it with fake debt recovery costs it would still be below the £200 threshold.
So, all you have to do is just ignore it. Ignore all debt collector letters, no matter how scary as they are powerless to do anything. They are designed for the low-hanging fruit on the gullible tree to poop their pants and pay the, by then, inflated charge. Eventually, they give up.
As for the £40 admin fee from your hire company, you'll have to read the small print on your hire agreement to see if they can charge you that fee for a speculative invoice from an unregulated private parking company. As it is not a "fine" or a "penalty", unless the agreement specifically mentions "parking charges", they shouldn't have charged you. All they had to do was give Smart the Hirers details and copies of some documents.
