It gets even better.
http://www.ntlworld.com/toolsservices/forms/upgrade/
NTL do not even admit that the Terayon modem is not capable of the full 10Mbit service!
Given that those with these older modems are long term customers who have been paying NTL money for probably 4 or more years, it seems madness to try to charge them when the equipment that NTL rents to them is no longer capable of providing the service.
NTL do say elsewhere that if equipment needs replacing then NTL will do so, and in this case it does need replacing, or we should only be paying 8/10ths of the price when the terayon maxes out at 8Mbit