Where are those signs in relation to where you parked?
Neither sign is capable of forming a contract because:
• No Offer: A prohibitory sign ("No Parking") does not make an offer. Instead, it forbids parking, so there is nothing for the driver to accept.
• No Acceptance: Parking in breach of a prohibition cannot constitute acceptance of a contract. A breach of rules does not create legal agreement.
• No Consideration: There is no exchange of value. The landowner offers nothing in return for the driver's alleged "agreement" to pay a penalty.
• Penal Nature: The £100 charge is a penalty for non-compliance, not a fee agreed under a contract. Penalties are unenforceable in contract law unless they serve a legitimate interest (e.g., in ParkingEye v Beavis), which does not apply here.
In short, prohibitory signs do not invite a contractual relationship.
Why do you think you should pay up? Did the driver breach a contractual agreement? If you think so, should the Keeper who was not the driver be liable for that alleged contractual breach?