This meant a couple of changes when setting up.
1. Use new cable supplied by BT to connect from phone socket directly to HH (no need for modem any more)
2. Remove red ethernet cable between HH and modem
3. Connect laptop by ethernet to HH
4. Change laptop router address to 192.168.1.254 (the default on BT HH)
5. Change laptop IP address to one in the 192.168.1.xxx range
6. Access HH admin screen (192.168.1.254)
7. Change the default BT wireless network addresses to those used with previous HH
8. Change the password to the previous too (this avoids having to go to every single device and change them all manually)
9. Change router IP address to that used on the old HH 4
10. Change DHCP accordingly.
11. Reboot HH
12. On laptop, change back to old router and IP address settings
Success!