I believe I have found the cause for this and the solution.
during the upgrade to 7 it lists a possible problem, in that the MAC address might change for the Linux Bridge
(total mumbo Jumbo to me)
However when I looked at the one node which did still work using "ip address" in the shell, I see this
Note the MAC address of the Network Card is the same as the MAC address of the Linux Bridge vmbr0:
On a node which
doesn't work then MAC address for the Network Card is the same but for the bridge it is different.
Note one ends in 33:88 and the other ends in 3a:2e
So to fix this you need to go to your router. (IMHO: I found it easier than trying to find where and what I was looking for in the linux system.)
On the router find your BIND IP TO MAC section and change the MAC from the one shown in the Network card section ( ends in 33:88) to the Bridge MAC address (ends in 3a:2e)
Migrate all the VM's to another node (if they are critical)
Reboot this node
Migrate the VM's back.
problem solved
Thanks Damon