I gather that some things changed in October - eg the 'keeper' is now liable for Parking Charge Notices (ie on private land), rather than the driver. Has your previous advice changed, which was ignore the notices or offer a notional £10. The company involved have been referred to as 'notoriously litigious'.

I parked (someone else's car) on what looked like a public road with double yellow lines on one side and white parking bays on the other. It turned out to be a private road, but I didn't spot the signs until I returned to the car.