Overnight the issue happened, a couple of other things to note:
While KVM worked fine and icon remained solid in config, the seamless desktop was broken.
1. they have Virtual Box Host only adaptors, now disabled but were active (on all 3 machines - so hard to see this was the issue, but disabled to rule them out). The RJ45 is disabled now too on the primary, so only wifi on all machines.
2. flushed DNS on both pri and problem sec, problem remained
3. restart Multiplicity processes on problem sec, problem remained
4. in pri config screen moved the machine positions and then moved them back (all in one session, not exit/save or anything) and while still in that screen the seamless returned. The primary process stack was NOT restarted.
I will try and reduce the actions to narrow the bit that fixes it, and will drag icons on pri config between/before each action to see if that is part of the seamless restore.
Hopefully by identifying the action that fixes the issue it might help show the route to a solution.
I will then start to restore virtual machine interfaces and then finally ethernet to see if one of these is causing greater frequency of failure.
Any comments appreciated