The BBC have announced the times that the preview will be available to see via the red button feature.
Sky/ Virgin media
Tue 21st December, 5:30pm-6:00am Wed 22nd
Wed 22nd December, 5:30pm-12:00am Sat 25th
Freeview:
Tue 21st December, 5:30pm-11:00pm & 11:40pm-5:50am
Wed 22nd December, 5:30pm- 6:20am Thurs 23rd
Thu 23rd December, 5:30pm-7:45pm
Fri 24th December, 4:10am-5:50am & 5:30pm-8:15pm & 11:25pm-5:50am
Sat 25th December, 3:10pm-11:00pm