The second episode of the series, which saw Johnson tracking down his great-grandfather, a radical Turkish journalist, and investigating the supposed French aristocracy of his granny, attracted 28.5% of the 9pm-10pm audience.
ITV's 'Taggart' came in second in the ratings battle, grabbing 3.1m viewers and a 12.8% share of the 9pm-10pm audience.
The episode, called 'Judgement Day', saw the team investigate the suspicious death of DCI Matt Burke's elderly father.
'Taggart' beat 'Big Brother', which pulled in 2.8m viewers on Channel 4 and an 11.5% share of the audience. Including Channel 4+1 ratings it managed 3.2m viewers and a 13.4% share.
Last night's instalment saw the remaining housemates attempting to replicate Michael Jackson's 'Thriller' video and discovering that Lisa, Sarah and Nicole face the public vote on Friday.
The final part of BBC Two's 'House of Saddam' drama, telling the story of the rise and fall of Saddam Hussein, drew 1.5m viewers and a 6.1% share of the 9pm-10pm audience.
It narrowly beat Five's 'Rory & Paddy's Great British Adventure' which attracted 1.3m viewers and a 5.3% share of the audience.
The second episode of the series, which follows comedians Rory McGrath and Paddy McGuiness as they travel the UK in a camper van in search of strange sporting events, saw the pair trying out axe throwing and caber tossing in the north of England and Scotland.