Ok, before we go down this route again (trust me, this discussion has been done to death.....).....
NTL don't block port 6881.... They block very few ports and none of them are connected to any bit torrent / P2P programs. There, that's that said.....
But, you may find you get better speeds by specifying a port somewhere on the high 49000's (something like 49878 for example).
Remember that if you change this port you will need to allow the new port access through your firewall in the same way that port 6881 is at the moment.
I don't think you will see much improvement though (although it is certainly worth a try).
Sometimes torrents just go like this, when they do there's not a lot more that you can do about it.
It also often depends on how long the torrent has been running for, the longer you are connected to it the better chance you have of establishing a connection with someone who is uploading at a decent speed.
Try the port change above if you feel like it, but this may be something that you just have to live with on this torrent - sorry
Oh, you're not downloading any other torrents at the moment are you?