Please stop referring to it as a "fine". A poxy, ex-clamper, unregulated private parking company is not an 'authority' of any kind that can issue "fines" or "penalties".
It is nothing more than a speculative invoice for an alleged breach of contract by the driver. Do not dignify them with the idea that their PCN is a "fine".
As for the failure to "invite" the Keeper to pay the charge, there can be no implied obligation. They either invite the Keeper to pay or they don't. In this case, they don't.
Many operators include something like this to overcome the issue:
You are now invited to:
(i) Pay the charge; or
(ii) Provide the drivers name and address for service and pass this notice to them.
They have chosen not to do this.