to Cable Forum
As Handyman has indicated, you can only get broadband from ntl from a Set Top Box, or Cable Modem. The modem can support faster connections that a Set Top Box, but realistically, anyone with a 2 meg connection via modem or STB should find it more than ample for online gaming.
A splitter will not be of any use whatsoever. You cannot directly connect a router to the cable network. A router just does not know how to speak to the cable network direct.
It you are really convinced that the STB is the problem, you can ask ntl to install a seperate cable modem. They would split the cable to do this so you can still get your TV services. You'd probably have to ask for an upgrade to the 10meg service and pay a £50.00 fee to get the modem.
As a 2 meg service should be very OK for gaming, what exactly are the problems you experience? Someone may be able to assist. Is the problem with your router setup? What happens if you remove the router and just connect the PlayStation to the STB (you'd need to power off and back on the STB when swapping connections)?