Why are you even considering the "mugs discount"? What has been received by the Keeper is a speculative invoice from an unregulated private parking company for an alleged breach of contract by an unknown driver.
Why on earth would you just pay an old invoice because it offers a discount? If you follow the advice here, you will not be paying a penny to ECP.
These firms rely on you being low-hanging fruit on the gullible tree who can be intimidated into paying up out of ignorance and fear.
The Plan A should always be to try and get whoever contracted ECP to get them to cancel it. Morrisons are extremely good at doing this and all you need to do is visit this site and fill in the form:
https://www.morrisons.com/help/form/contact-us/car-park/car-parking-charges/i-have-a-car-parking-chargeIf that is not successful (very rare) then you move on to Plan B which is an appeal, which we will advise on if you don't have a result before Thursday 2 October.
Even if this went all the way to a county court claim, I can tell you with greater than 99% confidence that it would never reach a hearing as long as it is defended with our template defence as it would be struck out or discontinued.
So, ignore the stupid mugs discount and proceed with Plan A for now!