The revival of the classic sitcom attracted an average of 9.43m (39.9%) at 7.45pm, according to initial overnight data.
EastEnders averaged 7.22m (31.3%) as Phil discovered Mick's identity at 7.15pm, with a further 591k (3.4%) tuning into the BBC Three repeat at 10.30pm.
David Walliams and Miranda Hart's comedy Gangsta Granny drew 5.79m (26.6%) at 6.15pm, while the first episode of period drama Death Comes to Pemberley took 5.90m (26.5%) at 8.15pm.
Later in the evening, the Boxing Day edition of Match of the Day pulled in 4.01m (27.3%) between 10.30pm and midnight.
Over on ITV, 5.08m (21.3%) tuned in for Emmerdale's hour-long special at 6.15pm, which saw Declan discover that Megan was trapped inside Home Farm.
The network premiere of Harry Potter and the Deathly Hallows: Part 1 was seen by 3.67m (14%) between 7.15pm and 10.05pm.
I Am Britney Jean: Britney Spears in Las Vegas followed with 743k (3.4%), as well as a further 40.5k tuning in on ITV+1.
BBC Two's highest-rated show of the night was University Challenge, which took 1.22m (5.1%) at 7.45pm, and Idris Elba: King of Speed drew 0.93m (4.2%) at 8.15pm.
On Channel 4, Independence Day kicked off primetime with 1.03m (4.5%) and Jimmy Carr's Big Fat Quiz of the Year attracted 2.23m (11.2%).
Michael Bublé's Christmas Special led the night for Channel 5 with 1.18m (5.1%), while the first round of World's Strongest Man 2013 pulled in 1.05m (4.7m).
The most-watched film on Boxing Day was Cars 2, airing to 3.53m (21.7%) on BBC One at 4pm.
BARB ratings data supplied by Attentional.