Just a little update. I'm at the dealership now picking up the car. They've flushed the coolant, changed the thermostat, and put a new hose on the turbo coolant send as it was leaking.

They say it's passing a tk test and they can't find any other problems with it.

I'm a bit mystified tbh.
Needless to say I'm going to drive her pretty hard over the next week or two and any issues at all she's coming straight back.