Well in practice there is nothing you could say that would persuade TFL to cancel the PCN voluntarily, this will have to go to the tribunal so that they can be forced to. As you can't take back what you said in the informal representations, you might as well send something short and to the point:
Dear Transport for London,
I require you to reconsider my informal representations afresh.
Yours faithfully,
It looks like you can't make representations online, so you might have to send a letter in the post, in which case you must keep a copy of what you've sent and you must get proof of postage (free at any post office counter).