Maps of the Parliamentary Estate can be found on the Parliamentary Intranet. Go to http://intranet.parliament.uk/access-buildings/access-security/maps/
If you do not have access to the intranet you can view a couple of these maps which show the parliamentary estate and surrounding area on pages 75-77 of the W4MP 2010 guide http://viewer.zmags.com/publication/0ce15b66 .