"On the motorway last weekend on a hot sunny day (over 25C) the temperature gauge was around 90 but towards the end of the trip with a bit of town driving it was showing 100. What should the reading be on a hot day on a motorway run? I thought the airflow to the radiator at around 65 / 70 mph should be good."
This sounds about normal to me. My 2014 GDI Plus 4 did run hotter when it was new and I guess the engine was a bit tight but it dropped back a little later. It may be worth replacing the rad with an alloy one if you considering any touring in southern Europe. They are more robust than the standard fit item, have a larger capacity, better cooling and they look better so it's a win win.
