I have exactly the same kind of problem - 10Mb on a NTL:Home 100 modem.
After reading around on the forum, discovered that the modems cant handle 10Mb, So phoned up VM and told the lady on the phone my 'research' i had done into the modems ....she didnt even question it or anything and agreed to send an engineer round and at the same time got offered a nice new deal
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As i understand it, the modems are their property and if your paying for a service which their property cant handle, which is a sub-standard service then your entitled to a modem that can handle 10Mb