Cannot upgrade to vSphere Applicance 6.0 - can't log into 5.5 appliance to correct certificates.

Help!

I'm trying to upgrade from 5.5 to 6

I've installed the Client integration plugin and I've started running the upgrade but I'm stuck at

4. Connect to Source Appliance because I get the error:

vCenterServer FQDN leeloo.castlehoward.local does not match DNS servers

I have checked my DNS setup and it looks fine (forward and reverse pointers are in place, IPs resolve) so I'm checking out this link

VMware KB: Upgrading from vCenter Server Appliance 5.x to 6.0 reports error: vCenterServer FQDN does not match DNS

which deals with an issue with the default certificates.

However, I can't log into the web interface of my source vSphere Appliance. It just won't accept my credentials, and I'm sure they're right because there are only two sets of credentials on the machine:

Root- which I use all the time

and

administrator@vsphere.local - and I know what this one is because I just altered it to make the previous part of the upgrade complete

I can only see these two accounts listed in Administration\Access Control\Roles for the vSphere Appliance.

So now I'm stuck! Does anyone have any ideas before I open a support request (and probably make myself look even more stupid!)