The Dominic West and Romola Garai series slipped to an average audience of 1.96m (8.7%), a significant fall from last week's 2.9m peak. An extra 168k watched on the BBC HD Channel.
Elsewhere on BBC Two, Restoration Home captured 2.63m (12.8%) in the 8pm slot, while Hugh Bonneville mockumentary Twenty Twelve picked up 69k (3.5%) at 10pm.
Richard Hammond's Journey To The Bottom Of The Ocean continued to be a strong performer on BBC One in the 9pm slot, enthralling 4.97m (22%), a slight 5k increase week-on-week.
ITV's biggest performer of the night was Cops With Cameras, which locked down 2.42m (11.8%) at 8pm, with a further 107k on +1. Homes From Hell followed at 9pm with 2.34m (10.3%) and a further 168k on +1.
The Sex Education Show attracted 1.05m (5.1%) to Channel 4 at 8pm and 170k on +1, while Undercover Boss collected 1.97m (8.7%) between 9pm and 10pm, with 315k tuning in on +1. Gordon Ramsay's Kitchen Nightmares USA earned 1.56m (8.9%) at 10pm and 171k on +1.
Channel 5's CSI triple-header from 9pm until midnight peaked with over 2.2m (9.7%) between 9pm and 10pm.
Chris Lilley's Angry Boys continued to prove a hit for BBC Three with 594k tuning in at 10.30pm (3.8%), while Geordie Finishing School For Girls was another success for the broadcaster with 730k (3.2%) watching at 9pm.
Overall, BBC One led primetime with 23.7%, ahead of ITV1's modest 13.9% (+1: 0.5%), BBC Two's 8.6%, Channel 4's 6.7% (+1: 0.8%) and Channel 5's 5.6%.
BARB ratings data supplied by Attentional