"Of course, I could unplug the dongle every time I get out of the car (some units have physical power buttons) but that's a faff I can do without."

Terence - forget the technology and the minor faff of disconnecting the dongle - just pay attention to your driving habits - and whatever the Yaris displays in the way of fuel consumption figures - a great aid to modifying one's driving style in these days of ever-increasing fuel prices.

A facility to display the on board diagnostics codes is useful when some nasty little warning light appears on the dashboard - you might then get some idea of what is wrong before risking being ripped off by some cowboy garage trying to bamboozle you. Anything more than that is potentially a dangerous distraction.