I just found out about the 2008 Heritage Open Days event directory.  It is now 'live' and is being constantly updated.  

There are about  3,500 events taking place across England in the next few weeks, many in places that are not often open to the public.  There are many churches open, as well as historical sites, museums and parks.  It seems to be a very popular event and I have struggled to get much more information to tell you here, due to the website experiencing problems due to high demand!  However I am sure it is worth checking out for an interesting day out.

Here's where to start your search:   http://www.heritageopendays.org.uk/directory