I've had a couple of these in the past and totally ignored them.
If you respond,then they know they've got a fish on their hook.
My advice would be to ritually burn the letter and repeat the phrase "I will not get done for this" over and over again,out loud to yourself for at least 10 minutes,the longer the better,with real conviction.
They normally send about 3 or 4 letters in total threatening court action/bayliffs/credit rating damage etc but hold your nerve,repeat the mantra,and they will leave you alone.
Again I haven't got time to look, but recall that when it was being discussed to death up here someone pointed out that a legal test case recently has strengthened the hand of the parking companies somewhat?