Accessing any admin area returns user to login screen

Hi

Had everything working OK with the site before redelegating the domain.

Today nobody can login to the site, when you try to login it simply redirects to the login screen again with no errors.

Have tried the following:

Removed all plugins

Removed the theme

Checked site options in database

If you enter an incorrect password you get an error

You can reset a password OK

Manually entered MD5 encoded password into the database to test

I added a login panel to the sidebar on the front end. This allows you to login successfully and shows you the admin bar at the top of the page. If you select any of these admin options it logs you out and sends you to the login screen.

I can see the relevant cookies are being set when I login to the sidebar widget but as soon as you hit any admin link it sends you to the login screen and the cookies are removed.

I am at an absolute loss on this one, any ideas or suggestions most welcome.

Thanks