This HaBO comes from G.V., who is looking for this category romance:
It was either a Harlequin Presents or Mills and Boon contemporary romance, probably published between the 80s to the 2000s.
It features a divorced main couple. The heroine retains her married name, which I believe was Pennington.
There’s a long period of separation and then the story starts with him finding out that she’s using his last name and he wants her to change it.
Sadly, I don’t remember either of their first names, only that the hero wasn’t happy she kept the married surname.