The Lynda la Plante nine-part series, which stars David Hayman and Victoria Smurfit, attracted a 23.7% share of the audience, according to unofficial overnight figures.
It debuted with 6.5m viewers and a 27.7% share last week.
It beat BBC One's 'Fairytales', a modern makeover of the tale of the Emperor's new clothes starring Denise Van Outen, which pulled in 3.4m and 14.7% in same slot.
It was also competing with Channel 4's documentary 'Cutting Edge', which returned with 'A Boy Called Alex' -- a chronicle of a 16-year-old music prodigy battling cystic fibrosis.
It pulled in an impressive 2.7m viewers and an 11.6% share, a significant rise on its last series audience average of 1.5m viewers.
It was also above the time slot average for Channel 4 on Thursdays, which averages 2.4m at 9pm.
Including Channel 4 + 1's time-shifted audience, 'A Boy Called Alex' drew 2.8m viewers and a 12.1% share.
A double episode of Channel 4's 14-part cookery series 'Jamie at Home' suffered a slight dip on last week's figures in the 8pm-9pm slot.
The first 30-minute episode was watched by 2.1m viewers and a 9.3% share, while the second instalment followed with 2.2m viewers and a 9% share.
If the audience figures for Channel 4 + 1 are included, 'Jamie at Home' earned 2.5m viewers and a 10.8% share at 9pm.
ITV1's recently relaunched 'News at Ten' continues to trail behind BBC One's flagship news programme, but has boosted its audience from earlier in the week when it was hovering just above the 2m viewers mark.
'News at Ten' was watched by 2.8m viewers and a 14.8% share, while 'BBC News' won its slot with 4.5m viewers and a 23.3% share.
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