56 days applies to formal representations against a Notice to Owner under the Traffic Management Act 2004. Your PCN was issued under the London-specific London Local Authorities & Transport for London Act 2003, which has no limits, other than one of fairness. In the latter case, London Tribunals usually apply a 3 months limit for a reply, after which it is regarded as unfair and an abuse of process under the grounds of "the penalty exceeded the relevant amount in the circumstances of the case"