So a form of words challenging it as a procedural impropriety, in that there are no powers under the relevant legislation and regulations to charge a penalty as well as the original CAZ charge?
No not quite correct; there are no powers to demand the CAZ charge on the PCN, only the Penalty Charge.
All you need to do submit is a simple and short representation.
This should state that the regulations that apply to the Bradford CAZ are The Road User Charging Schemes (Penalty Charges, Adjudication and Enforcement) (England) Regulations 2013. You have been independently advised that the PCN is unlawful because there are no powers within the above regulations for the CAZ charge to be demanded in the Penalty Charge Notice, only the penalty charge can be demanded. This is confirmed by the Traffic Penalty Tribunal Case Number IA01249-1803 Decision dated 13/06/2018, Ajudicator Caroline Sheppard.
In her decision, she stated, (inter alia) that " There is no power in Regulation 7 for the PCN to require the road user charge to be paid in addition to the penalty charge."
Therefore demanding payment of the CAZ charge as well as the Penalty Charge is a procedural impropriety and the PCN must be cancelled forthwith.
What I suspect they will do is ask you to just pay the CAZ fee as they seem desperate to avoid the matter getting to the TPT. I wonder why !!!