Thanks for your reply. Actually, I think I may have fixed it but will have to watch something for a while to check it's ok.
It was originally set up with wi-fi but for a reason I can't now remember I put a powerline plug in (it took over an hour on the phone with a Virgin engineer to get my V6 and old Tivo to communicate so I'm a bit Tivo setup overloaded at the moment!). We then watched something on Netflix and the picture wasn't great. Have now taken the plug out and it seems ok but only time will tell.