I need help with the name of a book?!?!?

I have a book I am wanting to reread, however, I can’t think of the name of the book. Its about a married couple who have a child, I do believe they are British, and she doesn’t feel appreciated by her husband. And a ‘friend’ of hers husband starts giving her extra attention and is flirting with her. She starts believe he has a thing for her so she starts dressing up for him, pushing her husband away and doing what she can to impress him. However, one night she has a few friends over, her friend and husband (guy she has a thing for) and they have a kid, well there child is upstairs sleeping in the main characters sons bedroom. The couple goes upstairs to check on their kid and start talking about how the main character is a fat cow and how they can’t believe that she thinks that he likes her and the main character can hear this all due to the baby monitor.

Anyone have any idea what the name of the book is? Thanks!