I did the 4pm tour and arrived 10 minutes before. I'd recommend doing an earlier one to give yourself about 1.5 hours as the museum closes at 5pm (they do 5 sessions a day). The tour lasted for approx 40 minutes and it was definitely worth it. It takes you into the Grand Temple - it's pretty amazing in there. The guide was enthusiastic and informative. I found it to be very interesting to learn about the history of freemasonry and the building. It's something different to do in London and easily accessible.
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