The TSRGD do not apply to car parks, so they're largely irrelevant, here's a revised draft:
Dear London Borough of Islington,
I challenge liability for this PCN on the grounds that the alleged contravention did not occur. The PCN alleges an off-street contravention, so there is a requirement for the council to communicate the terms and conditions of parking created by the off-street parking places order to users of the car park.
As the location is off-street, the Traffic Signs Regulations and General Directions 2016 are irrelevant and are of no application. Ordinarily in an off-street car park there would be a tariff board outlining that the car park is subject to statutory parkings controls, listing the terms and conditions of parking, and stating that failure to comply with the requirements imposed by the parking places order might result in a penalty charge notice being served.
At this location there is nothing to bring the terms and conditions of parking to the attention of motorists, indeed here is nothing to advise motorists that they are within an off-street car park subject to statutory controls, nor is there anything to inform users that the bay markings and signs are anything other than merely advisory.
I had been given no information about the fact that the land in question is an off-street car park, or that a parking places order imposed any restrictions, let alone that a statutory penalty charge notice might be issued for failing to comply with the terms of the order.
In the circumstances the alleged contravention cannot have occurred, and the penalty charge must be cancelled.
Yours faithfully,
Make sure to keep a copy of the representation, and a screenshot of the confirmation page.