Thanks for the help.  In summary it works.    Granny's HG659 is connected from its WAN port to the main home H659.  I changed the VLAN setting in Granny's H659 as suggested to me.  Granny's HG659 has automatically connected and used the 192.168.3.xxx subnet and is IP numbers are been issued to devices on Granny's HG659 in this range.   Chrome Cast to Granny's TV is now hidden from the main house and verse versa.  This works well in this domestic situation.   I'm quite impressed with the HG659 as I did not think it would be configurable enough, but even with the Vodafone firmware installed you can get Admin rights and make the changes, NOTE the Vodafone login does not give many options.