In case you don't buy a weekly TV listings, according to the Radio Times the CiN Doctor Who material is split into two parts. "An exclusive treat from Doctor Who" is scheduled for the first half hour - 7.30 - 8.00pm, whilst the trailer to the Christmas Special is down as being in the 8.30 - 9.00pm slot. Naturally, being a rather chaotic live broadcast, timings are only ever approximate.
Don't forget this is actually a charities fund-raising event - not just some new Who. Donate by logging onto bbc.co.uk/pudsey or call 0345 733 2233.