While You Were Sleeping - S1-E2
Other mistake: When Sherlock was slapped, he later says he was surprised because the girl who slapped him was a "lefty." Then he rubs his right cheek and says it's leathery from all the slaps he's gotten, while the left is soft. This is backwards - anyone hitting him with their right hand would hit his left cheek.
Answer: I'm the same person who asked this question. I discovered it was Molly Price whom I recognized from Third Watch.
It should be noted Donna was Jim Fowkes‘ secretary. Donna killed Peter.