Ok have decided 4 degrees is about my maximum before having to take the car out of Eco Mode - can't believe the heating and fan speed difference when switched it to normal mode this morning! I would have liked a bit more heat in Eco mode, understand priority is keeping engine temp but it can get rather chilly!
Still it's returning impressive mpg for cold start shortish runs, about double the mpg of the new Merc C200 had before.