The Christmas Hypothesis by Anna Blix

Niklas has spent fifteen years at the North Pole.
He has dedicated his life to science, but is still waiting for the big breakthrough when he will finally get the recognition he deserves…
…And on top of that he has an embarrassing problem. Every year before Christmas, he receives a huge amount of misdirected post — children’s wish lists to Santa Claus.
When Niklas loses his funding he tumbles into an identity crisis. If he isn’t the brilliant scientist he has fancied himself, then who is he?
He follows one of the letters to London. There he meets Scottish reindeer herder Clare, who hates everything about Christmas…
…and his eccentric landlady Mrs Dollimore, who believes that those letters were not misdirected at all, and that Niklas is the real Santa Claus…
There’s only one way to find out! Together they come up with The Christmas Hypothesis.