Here are the links to websites that I use regularly
http://londonist.com/ A diary of daily events in London.
http://www.londonhistorians.org/ If you become a member of London Historians you will have preference to go on their events which often allow access to places that cannot be accessed as an individual.
http://www.cityoflondonguides.com/find The ‘find a guide section of the City of London Guides is particularly useful if you are looking for a guide who guides in a language other than English.
http://www.carolstanley.org.uk/pages/history.html This link takes you to City Sources a beautifully presented site consisting of links to other sites with everything you could possibly want to know about the City of London.