You
could do it yourself, but you would be violating ntl's terms & conditions, & if you screwed it up somehow you'd have to pay to get it fixed.
But, it is possible -
so long as you get decent quality cable with properly fitted connectors
Have a read of this thread:
http://www.cableforum.co.uk/board/sh...ad.php?t=13601
... and in particular, this post:
http://www.cableforum.co.uk/board/sh...2&postcount=28
If you do do it yourself, get the cable made up properly. Dodgy cable, & poorly fitted F plugs, can affect your service & that of your neighbours.