Thanks for that Doctor! I haven't had this car a week yet and had borrowed an identical 2010 Mondeo on the run up to buying the new one. That one's glow plug light always came on for the first start of the day, and then only flicked on subsequently.
For this 'new' (to me) car, the glow plug light came on every morning in the same way until yesterday when I noticed that it didn't. Later in the day the car 'refused to start' and the battery had to be changed since which it's started fine but the glow plug light didn't come on after 24 hours - though it's not quite zero degrees.
I'm an engine ignoramus but am keen to tackle any problems before I've had the car too long!