If they do issue a PCN, VWFS will transfer liability to the Hirer (I presume either you or your wife). VWFS will most likely also send you a copy of the Notice to Keeper (NtK) together with a worthless letter of authority to deal with the charge.
You cannot do anything until you receive a Notice to Hirer (NtH) from the operator, who you've not stated who that is, but I'm presuming Horizon Parking for now. Once you receive the NtH, you can appeal without identifying the driver.
You can bet your bottom dollar that they will not have fully complied with all the requirements of PoFA 2012 to be able to transfer liability from the unknown driver to the known Hirer. Whilst any initial appeal will be rejected, you will be able to appeal to POPLA who will take a more pragmatic view.
Without knowing the identity of the driver, they could never litigate this all the way to a court hearing because they would get spanked. However, that does not mean that they won't try of POPLA screw up.
There is also the interesting argument that the no return within 1 hour is also difficult to prove, if the drivers identity is not known and especially so in this case where two different drivers are involved.
For now, you wait for an NtH to arrive in the Hirers name and then come back and we will advise further.