So this is one of the stupidest PCNs I have seen in a while: out of the 23 photos on the council website there are no photos of any signs at all, so there is no evidence of a contravention. Of course you must not alert the council to this flaw in their evidence as you don't want to give them time to go back and get some photos, this should only be brought up on the day of any tribunal hearing.
In order to get the ball rolling I'd start by making a very basic representation:
Dear London Borough of Sutton,
I challenge liability on the basis that the alleged contravention did not occur, this is because I obtained a ticket from the pay and display machine, this ticket was visible on my dashboard and at the material time it had not expired.
Yours faithfully,
Of course the council will reject, but let them spell out their reasons and dig themselves into a hole. Send the representation online and keep a screenshot of the confirmation page.