Thanks Mike - as the Hub is in Modem Mode, I cannot see those settings.
The Nighthawk Router has the hub plugged into the WAN port, which is rated at 1gbps. However if I go to the statistics page for the router, it says the WAN port status is "100M/FULL" - whereas two of the LAN Ports are showing as '1000M/FULL".
I had tested the router using a different cable, and nothing in the setup has changed since I got the 500Mbps speeds earlier this year, but it appears this may be the culprit.
Thanks