It's pity you appealed already. If you had waited for them to issue a Notice to Keeper (NtK), you could also have them for breach of your GDPR under the Data Protection Act 2018.
So, where in your lease does it mention that you must display a permit to park in your demised space? Where is there any mention of you requiring to pay a third party, not party to your lease, £100 each time they issue you an invoice because they think they can scam you just because the managing agent decided they can put up some signs?
If you can, you should ask your landlord for a copy of the head lease they have and what exactly does it say about parking. As before, what it doesn't say about parking is equally important.
What has your management company said about these PCNs? They know you have a right to park there. They can get PCM to cancel these. Remind them that should they not be cancelled and this is litigated, they are jointly and severally liable for the actions of their agent. Ask for evidence in the lease that permits a third party, unregulated private parking company to require tenants or residents to display a permit and allows the third party to invoice resident/tenants for breaching a term on a sign that is not mentioned in the lease.
Point out that unless they can evidence having gone through the tribunal process of amending the lease in accordance with the Landlord & Tenant Act 1987 Section 37 5(a) or (b), PCM have no lawful right to issue PCNs.
PCM's signs cannot override the existing rights enjoyed by residents and their visitors, nor retrospectively and unilaterally restrict existing grants and easements where provided for within the landlord's lease. All modern leases include implied or express rights that cannot be overridden or interfered with, by a third party.