If you don't want to go wireless, you are prob best sticking with the cable.
You are not allowed to 'fiddle' with the NTL wall boxes and the internal modem on the STB will not be active on the network. NTL would charge you for any changes they make, prob about the same as buying wireless kit! Using a wireless router would have the advantage of adding extra security to you broadband, but it if is only temporary I would leave as is.
HTH &
to CF