I have further reconfigured my virtual switch infrastructure in my data centre. I now have two Cisco Nexus 1000V distributed 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 an ESXi dVS (distributed Virtual Switch) which is shared (ie: "distributed") between the 2 DCs.
This is all illustrated in the attached diagram: