Plusnet are not such a great bunch. See them listed here ISP's known to BLOCK or Filter p2p or specifically Limewire. They might have noticed you using a lot of bandwidth & then applied p2p bandwidth shapers / port blockers.

BTW if you're using XP, then you will have the XP firewall. Configure it here Firewall configuration instructions (click on link)

Then see if the brick wall disappears. Also after that, try downlding a magnet link & tell us what happens in regards to connecting & speed http://www.magnetmix.com/video.shtml