This is by H28 on onedirectionfanfiction.com it's wonderful and I hope you like it as much as me. Recommended for 17+

'Not once did I think would end up in a stupid supermarket buying condoms for my boss's son!' ... Ella Lane became the personal assistant of one of the worlds most respectable business men as soon as she graduated high school, though she didn't know the extra baggage that would come with the job being his son, Harry Styles. He's flirty and charming though Ella can see right through his games as she has become accustomed to it over the past three years in which they soon develop a playful friendship. But what happens when after all this time, one of them begins to feel something just a little bit more than a mutual friendship....