Some users are experiencing problems when they try to access Google Sites using Virgin Media, Sky or Blueyonder accounts. Presumably this is something to do with the fact that these are (afaik) gmail re-badged! To resolve this, follow the instructions given below.
Some users have been experiencing problems accessing their site, after signing-up using email addresses provided by Virgin Media, Sky or Blueyonder. To access your website and edit in Google Sites, please follow these instructions:
You can now sign in to Google Sites using the new email address specified (alongside your original password), and can edit and improve your business website.
*We recommend signing up for a government-backed MyGuide email address.
Read one users' experience and how they resolved this issue here: https://productforums.google.com/d/topic/sites/gmSEtHvi2G4/discussion