Please stop referring to the PCN as a "fine". I will give you £100 for every occurrence of the word "fine" that you can show me in the NtK you received.
What you have received is a speculative invoice for an alleged breach of contract between the driver and G24. The invoice is actually for £100 with a £40 bribe discount if you make their life easier by not appealing or challenging the alleged debt you owe them.
As G24 are not members the BPA, the supposedly independent appeals service, IAS, is nothing but a kangaroo court whose main purpose is to look after their paymaster members interests, not the appellant. What this means for you is that you are either low-hanging fruit on the gullible tree, ripe for picking by B24 or... you are not and you are prepared to take advice on how to fight this.
It's your money and time. If you are prepared to follow the advice, in a worst case scenario, there is no danger of a CCJ or any damage to your credit rating and you risk paying around £100 plus £35 in court fee and £50 in legal fee. In the best case scenario, you don't pay a penny and get a great life lesson in standing up for your rights and how the civil law system works.
Even if it went as far as a county court claim in the small claims track, there is every likelihood that they would discontinue before any hearing. They hope that you are gullible enough to simply pay them once they start threatening legal action.
To simplify the above... you don't owe G24 a debt unless a judge says you do. Do you think you owe an unregulated private parking company a debt or not?
