BBCA tends to show their scifi programming on Saturday nights. With Waters of Mars and End of Time 1 airing on back to back Saturday nights, it's a good bet part 2 will air Sat night Jan 2. I imagine the delay in an announcement has to do with BBC scheduling games, but I'm 99% sure part 2 will air Jan 2nd.
For BBCA that would make sense, given that part 1 is airing on the 26th anyway and the US doesn't tend to go in for the whole Boxing Day "double holiday" the way the UK and Canada do. Also, airing it on the 2nd should remove it from competing against all the New Year's Day football games.
Alex