Communicate by email is best as this provides proof of sending and receipt.
What the lease doesn't mention is equally important as what it does say. In the absence of any requirement to display a permit mentioned in the lease, not displaying one is not a breach.
You may have displayed one in the past as a courtesy, but doing so, does not imply any obligation to do so. Additionally, as there is no mention on your lease that a third party, unregulated private parking company, contracted by the landlord, can issue you with speculative invoices for an alleged breach of their third party contract, therefore, you cannot be liable for those alleged debts.
PCM are notorious for trying this on in residential car parks. Sadly, the vast majority of residents at these PCM managed car parks are ignorant of their rights and how they are protected by their leases. PCM rely on their income from low-hanging fruit on the gullible tree who have been failed by the failure of the education system to give them even a basic understanding of civil law.
Simply put, your lease has supremacy of contract over anything that is later introduced. A landlord cannot simply change or alter what is in the original signed lease. They have to go through a whole legal process involving a Tribunal and the requirements of the
Landlord and Tenant Act 1987. Just have a read of section 37 5(a) or (b) to understand.