This is really good, you can walk all around for as long as you like, and just turn up anytime. It's open 10-6 Sun-Thur, and 10-10 Fri+Sat, all year round. Nearest tube/train is Blackfriars, about 5 min walk, other stations like Southwark are close. It's right across the Millennium Bridge from St. Paul's! A really good experience. You can view both from open air, and from inside glass for colder days.