Did you follow the advice of @M456 as noted above, which should have been your Plan A?
Yes I did I have called that number several times in December - they give you a menu and there isn't an option for 'cancelling parking charges' so I tried various departments before giving up. I spoke to several customer service agents who all said they can't help.
HOWEVER! Your reminder did help in that last night I happened to be driving past the store and just popped in and asked politely what we could do.
They have cancelled the charge.
The lady I spoke to said if I can show I did spend money in store that day and stayed for less than 3 hours, they'd cancel it no questions asked. The parking charge letter had my entry and exit times and it was well under the 3 hours (although I now know the signage states only 1 hour!). She wrote on the parking charge letter itself her name, the date, and that she'd seen evidence that I'd been a customer - she said a receipt is best but she was happy to just see the transaction on my banking app to prove I had been a customer on that day).
She then kept the letter after I took a photo of it for my own records. She placed it on a whole pile of letters that looked the same so I guess this is a common thing at that Asda. I asked if they would contact me and she said no, she doesn't think so, but that I should be assured it's been cancelled. I suppose it will take a few days for the Horizon Portal to update.
So I think the advice should be go direct to the store if you can. Thanks again everyone for your help. We can call this one a success.