It's a really neat idea for a series and the first episode definitely made me want to watch more. There's another two tonight so I'll get the second from last night watched beforehand.
Spoiler:
And of course, I love the addition of a murder mystery plot. BBC One is really delivering my whodunit needs this festive season!
Having Marley murdered on Christmas Eve was a nice touch since in A Christmas Carol that's the night he died. And of course, him dying at the end of the very first episode is fitting for a character who was "dead to begin with".