As previously reported, the production runs from 24 October to 23 November (previews from 17 October).
Directed by Edward Hall, the play centres on a couple whose attempt to have a quiet, child-free weekend in Wales soon descends into chaos.
Tamzin Outhwaite returns to Hampstead following Di and Viv and Rose in the spring. Her other theatre credits include Sweet Charity and Boeing Boeing (both West End).
Robert Webb, one half of the award-winning Mitchell and Webb double act and star of Peep Show, made his stage acting debut in 2008 in Fat Pig.
Paisley Day is best known as a comic actor, with recent credits including Twelfth Night at the National, and Private Lives and The 39 Steps in the West End.