A corner of Martha's mouth twitched up into a smile. "So did my mum really write you a letter telling us we couldn't have a Nintendo 'cos it'd rot our brains?"

Anyway, even a time traveller would have problems with doing all that work - not to mention crossing his own timeline. But Martha didn't contradict him.

"Still north." Martha tapped it against her hand for a moment. "D'you think it's stuck? 'Cos I remember being at the North Pole, and not only was it more than a little cold, but there definitely was not any sort of Santa's workshop there."