I think from press reports over recent days we can firmly conclude that having Gavin and Stacey on Christmas Day was something of a last-minute decision. It's pretty obvious the BBC had planned to air Victoria Wood alongside the Royle's, but parachuted in G&S and possibly Catherine Tate too from Xmas Eve after they'd seen the state of the show.
Interesting to see opinions about Doctor Who varying widly and speculating whether this will impact on the numbers. Tonight encapsulated a lot about Russell T Davies' series: it was massively hyped, attempted to be hugely epic, had lots of whiz-bang effects, several memorable scenes, and was held together by a wafer thin story.
The specials and finales are usually a bit of a let down - but they always capture the attention, and therefore the ratings. Returning villains, returning companions, stunt castings, logic-defying cliffhangers - RTD has used them all to brilliant effect to make the biggest noise possible, even if it comes at the expense of artistic or storytelling merit. Regardless of the quality of tonights episode, its ending will still be a subject of huge speculation and discussion over the next week and will help, not hinder, the chances of the second part. Steven Moffat might write more fan-loved and critically-adored stories, but he'll do well to match the sheer populist appeal, and the subsequent ratings, achieved in the last five years.
I expect BBC One numbers to be broadly similar to last year (pushing towards 12m), with the roughly 500,000 extra HD viewers meaning a larger total audience.