the terrestrial arial needs to connect to the rf in, where virgin cable normally connects.
You then need to enable the tuner B (menu -> setup -> service Searching -> Tuner Config -> Tuner B and set it as DVB-T2)
Then exit back to service searching menu and select Autobouquets Maker -> Providers and enable freeview, now press green button to save and then run a scan.
I'm not in front of box with DVB T2 tuner so above may be slightly off as going from memory.