I've managed to work out why I was getting slow speeds with BitTorrent: if the tracker is on port 80, NTL's transparant cache intercepts the HTTP request and connects to the tracker from the NTL proxy server.
When the tracker gets this request, it tries to open port 6881 on the proxy rather than on my PC, thus other BitTorrent users cannot connect back to my machine and I get a slow download.
Has anyone worked out how to get round this?

Apart from using Torrents with trackers on other ports, that is.
Cheers