Nope you aren't supposed to be buying your own modem, and ntl won't install it for you.
The 200 is fine in most circumstances, that probably wasn't the fault to be honest could have been other issues. Harlow has been problematic historically.
So you've wasted money buying a nice doorstop, wasted your time calling tech support about the stolen modem (it wasn't the person who sold it to you's property to begin with, they remain property of ntl), and wasted your time posting here, no-one can or will help you.
It is a pretty doorstop though isn't it?