The previous problem I had with picture/sound break-up whilst watching live football matches on BT HD seems to have been caused by slow broadband speeds between router/Home-Hub and BT TV box. Transmission between these 2 was via BT Powerline adaptors due to Home-Hub and BT TV box being in different rooms. Moved Home-Hub adjacent to BT TV box and connected via direct Ethernet cable. Problem has not reappeared over the last few weeks. Thanks for assistance/suggestions provided by BT Community.