Firstly, goto
http://192.168.100.1/P_downstream - that should tell you your signal levels, if you could post those back to us (you'll have to reset the cable modem first, and enter root and root for the username and password).
Apart from that (if the signal levels are low), there isn't much else - NTL are disgraceful to be doing this. One thing to check are the connections between the cable modem and the 'in' coax cable (make sure it's screwed in tight), and make sure the other end of the cable is screwed into the white isolation box tightly.