For future reference all you needed to say was:
Dear London Borough of Brent,
The alleged contravention did not occur as there was a valid parking session in force at 11:14 am (I attach a copy of my parking receipt), therefore please cancel the PCN.
Yours faithfully
Everything else is unnecessary waffle. This has nothing to do with mitigating factors, and it's not down to the council whether this PCN is cancelled, the PCN must be cancelled whether they like it or not because on the facts the contravention stated on the PCN did not occur. As often happens, you rushed into making a representations that includes all sorts of unnecessary self-incriminating statements, but thankfully on this occasion none of that matters.
Let us know when you hear back from them.