I'm on ADSL Home Surf at the moment and I have a simple question ... Why does plusnet's speed test always show me at 480 + kps when independent tests such as dslreports and broadband wizard can show as little as 250 kps?
Having migrated from Freeserve I am seeing a marked difference when video streaming for the worst
You may also want to try the BT speedtest which uses an even shorter path to the test machine than PlusNet becaise it is within BT's own network.: see ADSL: Using BT’s Speed tester service
What MTU values are you using. Maybe using a different MTU value in yoru PC (and the same one in your external router if you have one) and adjusting Windows RWIN value may improve your speeds. See MTU tweak.