You should be aware that the £40 reduction is known as the "mugs discount". It is there to make life easy for the PPC. Having to del with appeals and more, they'd rather their victims think along the lines of "I'll just pay it and be done with it" rather than know their rights and fight it.
You should be aware that this is likely to escalate all the way to a court claim, where you would ultimately win or, more likely, they would discontinue before any hearing, especially when they have a very weak case. Once any appeal is rejected, the "mugs discount" is gone. After 28 days, a debt recovery agent (DRA) will be in touch and will have added a fake £70 charge to the original amount.
DRAs can be safely ignored. They are powerless and cannot do anything, no matter how scary they make their letters sound. They are contracted on a no-win, no-fee basis, to try and scare the low-hanging fruit on the gullible tree into capitulating and paying into their scam.
The DRAs are not a party to the contract the driver is alleged to have breached with the PPC. They should be ignored. Never, ever, ever, enter into communication with a DRA in a PCN dispute.
The next phase, which may or may not come to pass, will be a Letter of Claim (LoC) giving you 30 days to pay or face a court claim. If/when a court claim is issued, it will have risen to around £255 which is made up of the original £100 PCN plus the fake £70 added charge plus £50 fixed legal costs and £35 court fee.
In the worst case scenario, if your defence of a claim was unsuccessful, the fake added £70 charge would not be allowed and it would mean you owed the PPC a debt of around £185-£200. Even if the claim was list, there is no danger of a CCJ as long as the judgment amount is paid in full within 30 days. It is completely expunged from the record.
However, the most likely scenario is that they will realise that you are not low-hanging fruit on the gullible tree and discontinue the claim and go off in search of other potential victims who know no better.
So, you have to decide whether it is worth fighting or not.