Thanks for all the help.
I reset the HG612 to factory defaults, then set the bridge to PPoE_Bridge, DHCP Disabled, Firewall Disabled, IGMP Snooping and Proxy enabled, UPNP Disabled.
Everything seems to work now - BT Youview, NetFlix Streaming, remote setting of YOuView recordings and all the Port Forwarded cameras and NAS systems.
Needless to say, config is now backed up!
Interesting statistic here;
The HH5 when 15m away from the socket was connecting at 22.4/4.7 and now with the HG612 connected beside the master socket on 1.5m cable, I am getting 27.3/5.7 connection speeds.
Wholesale speed tests also improved from 21/4.2 and are now 24.7/5.3.
Thanks again for the guidance.