I was at it six months ago, 'Sly' changed the LNB twice and the 'box' before they admitted it was the cable and renewed it. The cables aren't the strongest in the winds we have up here. And Sky came out to me three times and billed me for three service calls, but when I played up with them I got too of the billed calls returned. They always seem to me to try and get away with the easiest alternative, even though you have told them all the tests have been done and it is definately the cable.
Disconnect, check and re-connect the cable feed at the LNb first then try. If still faulty try using another LNb port. If all else fails, check the cable for joins, breaks and any water ingress. Replace if necessary.