I have further reconfigured my virtual switch infrastructure in my data centre. I now have two Nexus 1000V switches whose VSMs (Virtual Switch Modules) back one another up across two data centres. The switch fabric is constructed on two Virtual Ethernet Modules (VEMs), with VEM 3 in one data centre and VEM 4 in the other.
I have also configured a ESXi dVS (distributed Virtual Switch) which is shared (ie: "distributed") between the 2 DCs.
This is all pictured in the attached diagram