No, not quite
The 56 days is the mandatory response period for formal reps against a Notice to Owner under the Traffic Management Act, so doesn't apply to your mum's PCN which is under the LLA & TfL Act 2003
However, London Tribunals have made clear that delays of more than 3 months in replies to formal representations ca be considered unfair and grounds for cancellation of a PCN unless the authority have a very good reason.
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So, that leaves 30th June being 3 months from the latest date reps could have been submitted.
I won't poke the bear and hope nothing comes back in the next 10 days.