The episode, based on Christie's novel 'Towards Zero' and the penultimate outing for McEwan before Julia McKenzie takes over, netted a 23.8% share of the audience between 8pm and 10pm.
BBC One's airing of the film 'Miss Potter', starring Renee Zellweger as children's author Beatrix Potter and Ewan McGregor as her publisher, came in second on Sunday night attracting 5.1m viewers and a 22.1% share of the 8pm-9.30pm audience.
Channel 4's 'Big Brother' drew 2.8m viewers and a 12% share of the 9pm-10pm audience. Including Channel 4+1 ratings the programme reached 3.3m viewers and a 13.9% share.
On Saturday night BBC Two's 'Dad's Army' night, marking the show's fortieth anniversary, proved a hit with 3.1m viewers tuning in to see 'Don't Panic! The Dad's Army Story'.
The story behind the popular comedy, presented by Victoria Wood, attracted a 15.3% share of the audience between 8.30pm and 9.30pm.
It was sandwiched between 'Dad's Army Night: Jimmy Perry's Favourite Episode' and 'David Croft's Favourite Episode'.
The former attracted 2.6m viewers and a 14.3% share of the 8pm-8.30pm audience and the latter pulled in 2.8m viewers and a 14.1% share of the 9.30pm-10pm audience.
BBC One's airing of the film 'Bridget Jones's Diary', starring Renee Zellweger, won the prime time slot on Saturday night with 3.8m viewers and a 20% share of the 9pm-10.45pm audience.
'Big Brother' won the prime time slot for Channel 4 on Friday night attracting 3.6m viewers and a 17.5% share of the 9pm to 10pm audience as people tuned in to see Luke evicted from the house and Rex's girlfriend Nicole preparing to enter as a new housemate. Including Channel 4+1 it managed 3.8m viewers and an 18.5% share.
The second instalment at 10.30pm, featuring Davina's interview with Luke, and Nicole's entrance into the house through the storeroom as one of the housemates' special prizes, drew 3m viewers and an 18.4% share of the audience. Including Channel 4+1 it managed 3.2m viewers and a 19.5% share.
'Agatha Christie's Poirot', starring David Suchet, secured ITV 3.2m viewers and a 16.2% share of the 9pm-11pm audience.
It beat the first episode of the new series of BBC Two drama 'The Tudors', which attracted 2.3m viewers and an 11.2% share of the 9pm-10pm audience.
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