Upgrading from ESXi 4.0

Hi

I am not new to virtualization and have upgraded from previous version of ESXi before.  I have downloaded the following file

upgrade-from-ESXi4.0-to-4.1.0-0.0.260247-release

and when I try to use the host update utility it fails.  I am sure I am missing something simple, can anyone tell me what it is. 

thanks for any help offered.