Free Love by Tessa Hadley

It's 1967 and London is alive with the new youth revolution. In the suburbs, meanwhile, Phyllis Fischer inhabits a world of conventional stability.

Married with two children, her life is both comfortable and predictable. But when Nicky - a twenty-something friend of the family - visits one hot summer evening and kisses Phyllis in the dark of the garden, something in her catches fire. Newly awake to the world, Phyllis makes a choice that defies all expectations.