Humanity, ethics, morality is totally absent from council parking departments. All that counts to them is getting the money in. The words I use are 'venal and rapacious'
Their letter is the usual Fob-Off, sent in response to all informal reps, but its quite staggering in its denial of all human understanding. You might care to pass their letter on to your local councillor so he can see what the council are doing.
If you want to stand your ground, you now need to wait for the Notice to Owner. So, are you the owner, and if so, is the address on your V5C Registration Certificate up-to-date ?