St. Margaret’s church is next to Westminster Abbey. The present building, consecrated in 1523, is the third on the site and since 1614 it has been the church of the House of Commons. John Milton (among a lot of important English people) is buried here although all the tourists wait for visiting Westminster Abbey and forget St. Margaret.
The sundial is not old and it’s visible because it’s a blue cercle on the white walls of the tower bell. If you rest in the Parliament Square I’m sure you’ll notice it perfectly.