I did a very quick site inspection last month on a long layover in town. I had afternoon tea, which anyone can do, not just guest. It is in the lobby and can get quite busy. I was there (with reservations) on a Tuesday at 3 pm. I really enjoyed the selection. I found the biscuits a bit dry, but the chocolates on the top were stunning.
You can have tea in the main lobby area (it should go without saying, by the windows are best), the library, or the smoking room.