Kerry on "Editing database tables manually, but the changes won't stick"

This is related to this thread:

http://wordpress.org/support/topic/cannot-login-to-wordpress-suddenly

but I think it's another issue.

I've attempted to make the changes here:

http://codex.wordpress.org/Changing_The_Site_URL#Changing_the_URL_directly_in_the_database

But when I edit the URL in the database, within a few seconds it's changed back to the previous one, despite my saving it. Anyone know why this is happening and how to stop it?