OK so I managed to get one of our remote ESXi hosts connected to the vCenter Server through the WAN!
The issue was that even though I had a NAT rule for port 4567 to 443, which allows vCenter to see all inventory etc., I needed to do a 1:1 NAT of port 443 to 443.
So the issue I now have is that most of our remote sites are already using port 443 for remote web workplace and it is forwarded to the server, not the ESXi host.
Is there a way I can setup the host so that it looks at a custom port for SSL rather than port 443?
Thanks all for your help!