Read on here that when using vcds you should switch the ignition lights on but ther's no need to start the engine. Running tests can obviously take some time. 

 

Remember something in the back of my mind from the old days that doing this used to burn out the coil or something (memory not what it was)

 

Is it safe to leave ignition lights on for any length of time now on these modern cars then?

The ignition can safely be left on for as long as necessary when using VCDS. Switch the aircon fan off to avoid draining the battery.