I had the same problem (in the Salisbury area) which was ongoing for about 2 months.
They visit check your cables, check where your cable enters the building, check the point outside, and check the connection to the modem.
I had THREE engineers come to check this. (everytime it was the same old routine)
The third one said I needed a network engineer to do summit to the box up the road. Hey presto about a week later it was fixed
Luckliy I had managed to compile a list of the times the modem had dropped the connection.
Windows XP is quite easy for this.
Control Panel
Administrative tools
Event Viewer
Then select system
If you click type it will sort all the events.........
I listed all the ones with the red X
EG.
X Error 22/06/2004 10:45:26 Dhcp NONE 1000
I figured out that most of them tallyed up with when I had no connection so I sent them a list with all the Dhcp errors / Times / Dates
I know they have modem statistics but they will try to give you a percentage of your monthly payment rather than the whole lot.
Anyway presented with the information I had they refunded 2 1/2 months rental in full
But good luck coz from experiance NTL Broadband is great....... until you get a problem, then its a nitemare getting it sorted
