I had one ages ago at my gym car park - had about 10 or more letters, went from reminders to threatening to ‘ok, we’ll accept x’ or ‘y’ basically reducing the amount and trying to get something from me…
I ignored them all and eventually after about 6 months they foxtrot oscar’d and left me alone
Also, I’m not sure about current law but they were basically invoices rather than fines, and not generally enforceable because the amounts they ‘charge’ for the amount of time you park there are completely unreasonable - you can also try a different tact and offer a ‘reasonable’ amount to cover the time you were there, eg down the road they charge £2 for three hours so I’ll pay you £2 as a reasonable charge for the time I was parked, and I assume the matter is now closed - send the £2 to them (in pennies if you really want to piss them off) and leave it at that….
To be honest though, if you ignore them enough I bet they go away!
one other option is to speak to citizen’s advice - not sure how much they might be able to help…