Have you received a Parking Charge Notice (PCN) issued as a postal Notice to Hirer (NtH) issued in YOUR name (the Hirer), or is it simply a copy of the Notice to Keeper (NtK) that was issued to the hire company? This is a VERY IMPORTANT distinction and will affect how you must proceed with this.
In most cases of a Hire vehicle, these PCNs can be easily defeated. The unregulated parking operator has no idea who the driver is. The Hirer is under no legal obligation to identify the driver and should never do so. Only ever refer to the driver in the third person. No "I did this or that", only "the driver did this or that".
So, please read the forum rules and post a copy of what you received, suitably redacted of any personal details but leave ALL dates and times showing. Please confirm whether the notice you received is issued in your name or is it just a copy of the NtK that the Hirer received and forwarded to you?
If it isn't, the Hire company must transfer liability from themselves as the Keeper to you the Hirer. Once they have done that, they are then no longer liable and out of the picture. The parking operator is then obliged to issue an NtH to the Hirer in their name and we can then advise on how to easily get this cancelled.
READ THIS FIRST - Private Parking Charges Forum guide