BT Parental Controls can be setup to block some categories of website but is it possible in the HH6 to block a specific website?
I know that other routers can , often by hidden features like TalkTalk do , but is there anyway in the HH6 to do this?
No its not possible on the home hub 6. The code to do that was incorporated into the original home hub 1, but was never implemented.
You can block specific websites with BT Parental controls.