Until you actually receive a PCN in your name theres not much you can do. Assuming you actually receive it you need to come back here and post both sides of the notice to hirer. At that point you will probably have grounds to appeal as its unlikely the private parking company has provided all the documents required to hold the hirer liable. Do not identify who was driving to anyone, if they don't know who was driving and can't hold the hirer liable (even if it's the same person) they are stuck.
Validated parking rather than a simple max stay is a bit odd in a MacDonalds, especially given the confusion from the staff member, however it's just outside Heathrow where they get major problems with people using the airport.