All done and dusted! Used update manager and all went without a hitch. Thanks for the information!
EDIT - I don't like the Web Client :-(
Done 4.1 to 5.1 over summer hols.
As @FN-GM said it is pretty easy there is a guide from VMWare.
The only thing the guide seem to emit is the method to upgrade the hosts by creating a Upgrade base line.
Done the server and 11 blades over a couple of days then about a spent about a week updating all the VM tools on the individual VM's.
SSO installed first, Inventory database upgraded, updated vCenter, updated update manager, then update the hosts but the VIM Setup almost does it for you.
All went through fine no problems, don't forget to use a customized image if necessary i.e. for HP's and check the hardware compatibility guide.