If you are an ex-telewest customer then you're modem is on the wrong frequency.
for the scientific atlanta epc2100 go to
http://192.168.100.1/gscan.asp
for the webstar dpx100 go to
http://192.168.100.1/gscan.htm
delete starting frequency, type in 339000000
press enter, you should get message saying set successful
modem will reboot and should then be on the correct frequency.