Hi Julien (again) 
Here is the vswitch info for host 1:
Host 2:
Note Host 2 has an iSCSI Guest VM port group because we have a VM' that has iSCSI sessions initiated within the guest.
I have successfully be able to ping from the hosts to the array via vmk2 and vmk3 to the array addresses and vice versa.
We have 2 paths to the LUN:
Path selection of Round Robin is recommended by DELL as a best practice. One thing I did notice was that the vSwitch config for Host 2 was not set to MTU 9000. It was 1500 so I have now fixed that. It is set to 9000 everywhere else. DelayedAck is also turned off as recommended by DELL.
When the issue occurred, all the VM's failed over to host 2 except on 1 VM due to insufficient resources so I'll fix that up. I've attached the hostd log and hostd0 log. Issue occurred at 8.09am and 9.35am on 26/07/13
Thanks for your help!