Stopping of engine when vehicle stationary
12. - (1) An authorised person who has reasonable cause to believe that the driver of a vehicle that is stationary on a road is committing a stationary idling offence may, upon production of evidence of his authorisation, require him to stop the running of the engine of that vehicle.
(2) A person who fails to comply with a requirement under paragraph (1) shall be guilty of an offence and be liable on summary conviction to a fine not exceeding level 3 on the standard scale.
Issue of fixed penalty notice: stationary idling offence
13. An authorised person who considers that a stationary idling offence has been committed may, in accordance with Part 7, issue a fixed penalty notice to the driver of the vehicle.
What bothers me is the absence of any definition of "stationary idling offence". For fuel economy, I understood it was best to turn off your engine if you're going to be stopped for more than 2 minutes. But it seems you can get a ticket on a whim. This is typical of many regulations nowadays; you don't know you've broken them until you've been penalised.