One Day in December by Josie Silver
On a snowy December night in London, Laurie James looks out the window of her bus and sees him-- the man she's convinced, in that split second, is the one. Their eyes meet, he makes a move in her direction... and then the bus drives away. Laurie becomes obsessed, spending months scanning every face at every bus stop and every cafe in the hopes that it's him. But it never is.
That is, until her best friend and roommate, Sarah, introduces him to Laurie at a Christmas party a year later as Jack, the man of Sarah's dreams.