The device has a static IP, Windows Firewall turned off, and no other firewall running. In BT Smart Hub Manager -> My Devices, the device shows correctly as having this static IP, with an icon to indicate that port forwarding is enabled. Details of the port forwarding are also correct. In Home -> Advanced settings -> Firewall, the Firewall is Default (not Block all or Disabled). The computer is not in the DMZ but I don't think it needs to be (perhaps I'm wrong about this).
I am not sure that the link is even reaching my network, since neither this external port scan nor trying https://<my Home Hub Broadband IP address> in an external link check generate an entry in the Home Hub Event log (Home -> Advanced settings -> Technical log).
My BT Smart Hub is a new model (sent by BT in Jan 2020) and firmware was updated on 19-Mar-2020. The computer is connected to it via Wi-Fi over BT Whole Home mesh networking.
Any ideas on how to fix this gratefully received. Thanks.