so actually, in the 5 minutes since posting this i have actually resolved it.  Followed these instructions to unregister the plugin: www.virtuallyghetto.com/.../how-to-unregister-vcenter.html then re-registered using the username[NAME].com format, it worked
Hi there, I have upgraded my (Dell plugin for vcenter) virtual appliance to version 1.6.0.34 - and I now cannot add a new vcenter.  Initially i thought it was a problem with the upgrade, so removed my existing connections, now when i re-add - I get:  'Could not contact the given vCenter server due to wrong credentials. Check the username and password.' I know this account is administrator, as I used it in the previous version. (this is an AD Account - we don't use local accounts) Thanks  
Cannot add vCentre to Dell Plugin after upgrade to 1.6.0.34