just an update really... further testing is ongoing as it's proving a tricky one to pin down.
some sync issues can indeed just be the channel itself having problems and this is sometimes mixed up in the ongoing general issue we are investigating.
ITV1 HD shouldn't really be part of the issue as it's not broadcasting in 5.1 sound just yet. As the above poster points out, that was a ITV issue.
I'll update as soon as I get more info.