Have you called the helpline? I have BT as well & once I had the same problem. They told me there was a problem with the server. It was fine the next day. They also tried to charge me for option 3 when I only had option 1 ( the ********!) but apart from that I've had no major problems.
Thank you, but I dont think it's the server because it's been happening for about three weeks now. It's a new computer & when the Dell assistant comes on to analyse the system it shows there is no connection between either the computer & router or no connection between the router & the server ! It's only thrown me offline once today - up to now - but another day it happens about every fifteen minutes..