@olivierlambert Thanks, I have tried using xe-reset-networking and the emergency network reset function in the UI of XCP-NG and neither seem to work to reset the management Interface, after rebooting they still have no interfaces at all (which happened when the failed uplink bond was created).
I will probably need to wipe and re-install to get traction again.
Also I am confused with the MTU.
The servers we are using are configured (using a server profile template) that we also use with vMware, and the uplinks on those are all MTU 9000 both in the VNIC templates (ie hardware side) and in the distributed vSwitches in vCenter (equivalent of bonded interfaces in XO?)
If their was a problem with MTU, I would have though they would not work either.
I will keep at it, thanks for the help.

@olivierlambert Thanks, I see they are there