The hints Evie found at the Kenway mansion leads her to a Monument and she needs to Examine and locate the key to the Piece of Eden in London. Let's check the sixth part of Sequence 5: A Room with a View.
Evie and Jacob meet on a rooftop where he discusses her progress with the search of Piece of Eden and he wants to help her, but Evie avoid and asks Jacob to let her continue her hunt alone and meanwhile you just hunt the Templars down.
Start Examining the Monument with Eagle Vision, Gather some Rooks to help to distract the policemen and place your first slate in the base of the Monument. Make sure you don't start a Chaos and run everyone out, just keep it simple and easy.
Now Climb up the Monument to place the other slate in the Monument. Then locate the secret of St. Paul's Cathedral, you'll also have a side objective to perform a Leap of Faith in a Haystack while on a zip line.
Once you locate the secret of St. Paul's Cathedral, you need to solve the St. Paul's puzzle to locate the secret vault. Check the image below to get the puzzle solved.
As you open the Vault, Ms. Throne appears and demands the Key to Piece of Eden. You need to Fight with her and as you perform to kill her she will try to escape with the Key. This ends the sixth part of Sequence 5: A Room with a View.