apt-get update signatures were invalid

[R Kimber]

If I run apt-get update I get:-

W: GPG error: http://security.ubuntu.com hoary-security Release: The
following signatures were invalid: BADSIG 40976EAF437D05B5 Ubuntu
Archive Automatic Signing Key <ftpmaster at ubuntu.com>
W: You may want to run apt-get update to correct these problems


Is this a problem with the mirror, that will be corrected in due
course, something wrong at my end, or is there something more
fundamental wrong?

- Richard.

 -- 
Richard Kimber
http://www.psr.keele.ac.uk/