Thanks for the prompt response Sai, its very much appreciated.
Of the two portgroups effected, one has a single vmnic and the other has three (Yes, I know how that sounds)
We also run a very flat network on this site and none of the ports should be using a VLAN.
Rather than disconnect them i'll trace and check the switch ports they connect to. This should rule out incorrect VLAN configuration.
I have noted on both hosts that the portgroups with three vmnics have one showing "None" in the observed IP ranges and am investigating why this is, however as both hosts show the same I would expect to see the same issue when migrating to host2 as well as host1.
If neither of these methods give me any leads I will connect one vmnic at a time as suggested.