Everyone seems to have missed the obvious point: if you can park for free for up to an hour, then there was no parking charge due so you can't have failed to pay it, see John Carmody v London Borough of Redbridge (2210237357, 24 June 2021) (https://drive.google.com/file/d/1N2RYak_r55cwQFhiRznWqZqU8mhAfR5F/view).
@LondonTraveller84 just make a one line representation as follows:
Dear London Borough of Redbridge,
The alleged contravention did not occur because no parking charge was due for payment.
Yours faithfully,
Send this online and make sure to take a screenshot of the confirmation page. Maybe one day they'll come up with a contravention of failing to have a free pay by phone session or something of the sort, but until then these PCNs are clearly unenforceable. The council will undoubtedly reject but it should be a fairly safe bet at the tribunal, as long as you avoid a postal decision.