The latest episode, titled 'Tabula Rasa', managed 740k (3.5%) at 10pm - the lowest figure recorded for the series to date. The instalment originally aired on BBC Three last Sunday, and pulled in a fair 545k (2.6%).
The double-episode opener, which aired on BBC Two Saturday, January 9, averaged 1.42m (7%).
Earlier on BBC Two, The Virtual Revolution logged 920k (3.9%) at 8.15pm, then 1.85m (7.1%) tuned in for QI XL at 9.15pm.
Over on BBC One, So You Think You Can Dance pulled in 4.99m (21.2%) during the 7pm hour, then the results show grabbed 4.82m (20.5%) at 9pm. The National Lottery: In It To Win It interested 5.38m (26.7%) at 8pm, and 5.74m (25.1%) watched Casualty at 9.25pm.
Haiti's Killer Quake: Why It Happened took 1.02m (4.3%) for Channel 4, then the audience climbed to 2.13m (9.7%) during Independence Day, which aired between 8.30pm and 11.15pm.
Five's repeats performed as follows: two NCIS repeats drew 1.41m (6%) and 1.46m (6.1%) at 7.25pm and 8.15pm, then CSI and CSI: New York put in 1.13m (4.9%) and 1.66m (8.5%) at 9.20pm and 10.15pm respectively.
Overall, BBC One came top during primetime with an average share of 23.6% to ITV1's 18.5%. Channel 4 was third with 7.8% (+1: 0.8%), followed by Five's 6% and BBC Two with 5.2%.
BARB ratings data supplied by Attentional