Free Traffic Legal Advice

I'd appreciate some help with a couple of Notice to Keeper parking charges I received regarding parking in Cradlebridge Retail Park in Trowbridge.

The site operates ANPR cameras at the entry and exit of the site. They changed the parking period from 3 hours to 1 Hour in the months prior to the parking charges I received. We had no idea they'd made this change, it's also not obvious ANPR cameras are used here. My vehicle overstayed the 1 hour time on site by 11 and 12 mins on two occasions.

I haven't responded to the Notice to Keeper and I am now being chased by Debt Recovery Plus for £170 for each charge. I'm willing to fight the charges on the basis of poor signage and the historical 3 hour free parking period changing to 1 hour.

Advice on what to do next would be appreciated. Should I continue ignore the correspondence and wait for Letters of Claim or should I formulate a response to Smart Parking or Debt recovery plus. I tried speaking to Marks and Spencer who occupy the main retail unit on the site - their manager was rude and unhelpful. Amazing how many people are complicit to these nonsensical charges.