Your letter is unfortunately a waste of time
If the referenced law is the 1984 Act then the matter of two dates is irrelevant - that only applies to decriminalised parking under the RTA1991.
The 1984 Act was a dog's breakfast and allows councils to set charges individually. All of these and the restrictions and application of restrictions
must be contained in a TRO (Traffic Regulation Order) and it is to this that you must refer if you wish to challenge the legality. This is obtainable from the Highway Authority - usually the County Council unless this is a Unitary Authority (when it is the local authority) or a Trunk Road/Motorway (then it is the Highways Agency)