Saturday 16 March 2013 11.00am
The two nesting sites I know of have been in Marylebone Road and in the Barbican. Not sure how many nesting pairs there are in Central London.
Saw one around St Pauls about a week ago, they use the City churches as viewpoints and have seen them around St Pancras.
I am not sure they have ever nested on Tate Modern
but they do use it is a spotting tower to teach their young, so will probably return.