Does the Penalty Charge Notice (PCN) for a traffic contravention issued by a local council need to be declared to the insurer? Also, does this affect the premium?
To be certain you'd have to check the policy documents of the specific insurer, or just ask them. However large insurance comparison sites only ask about motoring convictions and penalty points, and a council's penalty charge notice doesn't count as such. So it's unlikely that insurers do want to know this information in general.
There's an overview of the different kinds of penalties here: https://www.rac.co.uk/drive/advice/know-how/fixed-penalty-notices/
A "Fixed Penalty Notice" is issued by the police or equivalent, perhaps via automated enforcement like speed cameras, and does result in a motoring conviction and often penalty points.
A "Penalty Charge Notice" is a civil penalty and won't carry any penalty points on your driving licence.