Not knowning much (anything!) about NTL digital TV cant really help, but that doesn't sound good, probably have to call for an engineer.
For your download speeds, go here for a speed test:
http://homepage.ntlworld.com/robin.d...speedtest.html
Should be around the 600 kbits/sec mark (according to my tests).
The one I just done ranged from 587 to 602
Cheers
Mikey