Don't just pay up, submit representations based on what has been discussed here, but don't expect the council to cancel the PCN. The dark heart of the decrminalised legislation is that it gave councils a Magic Money Tree, especially in big cities like London. All they do is reject virtually ALL representations, because they know that most people, (>95%) then just pay-up.
It all comes down to your attitude to risk. This is what the Earl of Montrose wrote: -
"He either fears his fate too much
Or his deserts are small
Who fears to put it to the touch
To win, or lose it all"
Unfortunately the Earl lost his head, but that doesn't happen at London Tribunals.