That noise margin is massive, When I rang Sky to find out if I had the best speed my line could handle after DLM they said they aim to get the noise margin as close to 7db as possible.
Mines currently sat at 8.2db, There is definitely a fault of some kind, Is this using the router Sky sent you? Tried different filters? Test socket?
Have you been in contact with Tier 2 support at Sky or just the basic support?