This is all I can think of - worded so you are just challenging on their evidence and not disclosing what actually happened.
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I am challenging the PCN because there does not appear to be any markings designating a parking bay where I parked.
I have checked your pictures and they confirm a lack of bay markings.
With no indicated parking places I do not think the alleged contravention occurred and I look forward to your early confirmation of cancellation.