i think its the ntl modem 100, request a 250.
they should swap it, as i dont think the ntl 100 modem can handle speeds of upto 10mb.
the speed your getting through it, i think is its max.
get a 250,
set it up through ethernet, your router should make no difference to the speed.
and then ...
go to,
http://www.speedtest.bbmax.co.uk/
and test your speed
good luck