I had the same issues, I upgraded to 10mb when it came out but as I had an NTL 100 modem I was only getting half the speed.
I phoned NTL and they came around the next day with a brand new NTL 250 modem ( the small blue ones ), the engineer fitted it and I instantly had 10mb and had no problems since.
You will not bee able to instal the modem yourself as an NTL engineer will need to register the MAC address of the modem which you will not be able to do, plus it's NTL's property anyway.
Phone NTL and explain you have on old mode that won't work properly with 10mb and they should send out an engineer and replace your old silver modem with a nice new blue one

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