Turns out I'm wrong on idling. It's an offence under the Road Traffic (Vehicle Emissions) (Fixed Penalty) (England) Regulations 2002. But it's only a £20 fine (and that only if you don't turn off your engine when told to) and I don't think I've ever heard of it being enforced. OTOH there's a consultation on bringing in bigger fines, but that will still depend on it being enforced.