With both shows concluding in the 9pm hour, Inside Claridge's gained the upper hand with an audience of 4.33m (18.5%), rising 200,000 on last week's instalment and 1.1m on December 3's debut edition.
The Poison Tree, meanwhile, shed a million viewers as 3.66m (15.6%) watched the two-part psychological drama's climax as well as 253k on ITV1 +1.
Over on BBC One, a repeat of a 2008 My Family Christmas special appealed to 3.01m (12.9%).
Channel 4's latest broadcast of Stephen Fry's Gadget Man drew 1.05m (4.2%) at 8.30pm (+1: 170k), retaining a comfortable lead over 5's Gadget Show which held a steady 737k (3%) across the 8pm hour.
Overall, ITV1 topped primetime with 21.4% (+1: 0.8%), beating BBC One's 17.5%. BBC Two earned third place with 10.5%, way ahead of Channel 4's 3.5% (+1: 0.8%), and Channel 5's 3.1% (+1: 0.2%).
BARB ratings data supplied by Attentional