I think I'm getting somewhere:
DHCP on the Superhub tuned off and the WAN port on the Draytek Vigor 2920 set to a static IP in the range of the Superhub i.e. 192.168.0.xxx
Firewall in the Superhub turned off. Don't really need it anyway since the Draytek is doing that.
Draytek router's bandwidth management / quality of service settings all turned off. WAN set to autobalance loads based on Draytek's autoweight policy.
Finally the WAN port for the VM Superhub connection, in the Draytek is set with an MTU of 1460.
Now I'm seeing around 5mbit/s upload