That's good news - everything works - well sort of.
All you need now is a combiner such as one of
these. If you ring up NTL customer services they
should be able to supply you with one - although the may insist that an engineer fits it. It allows you to combine together the RF output of your STB and the 'spare' leg of the splitter so you have a single feed to your booster.
You could achieve the same effect with another splitter wired in reverse. It would work but I wouldn't recommend it since part of the STB's RF output will pass back up the cable. NTL won't be happy and your neighbours will be able to see what you are watching late at night
Best of luck with customer services. If they won't play ball then there are other, more long winded, ways of achieving the same effect. Let us know what happens.