Starring Benedict Cumberbatch, the 90-minute show only averaged 955k (5%) between 7.45pm and 9.15pm. However, the audience rose from 838k (4%) when the programme started to a peak of 1.61m (8.2%) in the final 15 minutes.
Hosted by Chris Addison, Have I Got A Bit More News For You followed Painted with Words with a healthy 1.95m (9.6%), before Secrets of the Pop Song stayed solid with 1.01m (5.6%) from 9.50pm.
Over on BBC One, the return of John Barrowman's Tonight's The Night dazzled 3.9m (22.5%) from 7.10pm, while ITV1's Odd One In - starting at the same time - slipped to 2.7m (15.6%) and 155k on timeshift.
The National Lottery: In It To Win It was the most-watched broadcast of the day with 5.25m (27%) from 8.10pm, while in second place, Casualty took 4.97m (24.6%) for an episode titled 'Divine Intervention'.
Meanwhile, Penn and Teller: Fool Us and The Marriage Ref rumbled on with 3.05m (15.9%) at 8pm (+1: 246k) and 1.88m (9.4%) at 9pm (+1: 182k).
Channel 4's screening of The Simpsons Movie drew 1.29m (7%) from 7.30pm (+1: 301k), then Camelot mustered 1.1m (5.5%) at 9pm (+1: 139k) while NCIS (8pm), CSI (9pm) and Law & Order: SVU (9.50pm) pulled in 819k (4.3%), 880k (4.4%) and 1.25m (6.9%) for Channel 5.
Overall, BBC One easily led primetime with 22.4%, far ahead of ITV1's 12.4% (+1: 1%), BBC Two's 7.8% and Channel 4's 6.1% (+1: 1.2%).
BBC Three's coverage of Coldplay at T in the Park entertained 490k (3.3%) from 10pm.
BARB ratings data supplied by Attentional