I'm sorry to read about the Channel 4 being pixelated. I have been looking into the TV services and found that there are some signal levels that would need some adjusting. Once this is done, you should be able to have your services back as they're meant to be
I've sent you a 'Private Message', you can find this by clicking on the little purple envelope near the top right of the page.
Thanks for your message. I've just checked and it seems to have returned. We don't have a TiVo box, though, ours is V+. It's very strange that it happened, as all our other HD channels appear to be working fine, although we haven't recorded them lately, so have no idea if recording would have affected them in the same way.