Well, after reading this thread I thought I had better stick my head under the back of our one week old MG4 SE LR and check. Sadly there is a definite leak, a few drops on the ground, evidence of seeping out of the undertray and being 'blown' rearward across the bottom of the car. My best guess 'finger test' is that it is thin oil rather than coolant.
Top marks to our dealer though. Following a phone call, car immediately booked in for inspection tomorrow am. Dropped it off tonight and home with a MG5 loan car.
Fingers crossed it is nothing too serious and an easy fix....