Thanks for responding. It's been fixed now by a very determined technician from VM. He found that I had, understandably (?), switched on the HD/UHD option on the TV. As HD/UHD is delivered by the V6 upgraded box there was a conflict in that each unit was fighting for control of HD/UHD! This explains why we always had some difficulty with the basic V6 box and our TV connection to it.
Others may learn from this experience as did I.