Would this include dropping or picking up a trailor on a public highway, leaving engine running building up air whilst you hook up.
Simple yes or no, source if available.
No - because you are not leaving the vehicle unattended.
S107 Road Vehicles (Construction & Use) Regulations 1986 (SI 1986/1078)
two reasons given that appear to allow you to leave an enginge running make for interesting interpretation
(i) drive machinery forming part of, or mounted on, the vehicle and used for purposes other than driving the vehicle; or
(ii) maintain the electrical power of the batteries of the vehicle at a level required for driving that machinery or apparatus.
So presumably leaving the engine running to operate the heater or to charge the battery or to build air would all seem very valid reasons - but do take proper professional legal advise before doing so
Don’t think that would cover picking up a trailer.
weeto:
Don’t think that would cover picking up a trailer.
probably not.
But the argument would be the engine was left running to power the heater to keep the cab warm - the heater is equipment used for purposes other than driving the vehicle, same as keeping the battery charged so as it can power the radio - heck, providing you are building air so as you can clean your cab out with the air duster or blow your bikes tyres up then you may have an argument not to accept a fine.
Laws are never black and white, they were designed by clever lawyers with built in ambiguity so they can be argued over for ever more, keeping themselves wealthy in the process.
I can’t see you getting away with the excuse that the engine had to be left running while coupling up to a trailer. Even if you required the PTO to be running for hydraulic power to the trailer for some reason the unit could still be parked engine off, connect lines and then return to cab to engage PTO and run engine. [EDIT: unless the neccessary controls that have to be operated are on the trailer] If the battery is suspect or battery guard has cut in then you should be in the cab to run the engine. It might be a bit chilly, but we aren’t living in Siberia where the diesel might freeze if you turned the engine off.
It’s just a debate between me and a workmate.
In my way of thinking it’s an offence to leave engine running whilst not in or very close to cab, any further and the the offence of “quitting” or leaving a vehicle unattended while engine is running would get you a ticket.not sure of the point of law but always play safe.a
If you do regular hook ups in public places, or drive a vehicle where for whatever reason the engine has to be left running whilst you unload, then the sensible thing to do is to have a cheap non programmed key copied and use that to lock the doors whilst the engine is running.
The sensible driver will also make sure the trailer brake is applied or red line is dropped, or some other way of making the vehicle more difficult for an opportunist (would be terrorist these day 
) thief to simply jump in select D and bugger off, possibly with the driver hanging off the back.
Car transporters, grab lorries (though there’s usually some handy brick outhouse workmates in steelcap boots with hammers and other potential weapons around when a grabber is working), skip lorries etc, have all for many years had to leave the PTO’s running whilst they did their job, never once was the engine running queried by a police officer during my time on the cars.
Our lorries only get turned off at the end of the day or when refulling usually.
We have pto driven pumps, so drive to the job, pump on or off, then drive away To the next one.
Been doing it 30 years without a peep from anyone.