The solution is not straightforward. Correct nslookup and ping responses are no guarantee of success.
The solution involves, inter alia :
1. refreshing/recreating the manual DNS entries, making sure that the domain suffix appears on the reverse lookup entry
2. flushing the DNS caches
3. creating a special local user in vCenter specifically for the Data Protection Appliance ( eg vdpuser ) with the administrative rights to manage the vDP, and then logging on as this user
This worked for me. Good luck - if everyone is having as much trouble with vSphere 5.1 as we are then that would explain why the support engineers are so snowed under.
Regards,
Eric