Recent press comment from Lidl said:
"We do encourage any genuine Lidl customers who have received a parking charge notice to get in touch with us directly via our customer services hotline number. Those customers who feel that they have been wrongly charged should then just provide us with proof of purchase and we will appeal the charge on their behalf.”
Call : 0800 977 7766
Showing Lidl Receipts
You may get asked by Lidl to show receipts demonstrating you are a Lidl customer (It need not be for the day you parked).
There is a page on Lidl's website that allows this here (https://www.lidl.co.uk/en/Contact-Us-4187.htm) Or you can just use this link to contact them if the phone number is closed or engaged.