I'm guessing you have just come from cable to dsl?
Try plugging your o2 box directly into the BT test socket. Locate the master socket, unscrew it and plug your filter in behind and connect your o2 box, then try a speedtest. By doing this you are isolating your household extensions and eliminating any chance of them losing some mb's.
I have a filtered faceplate from
ADSL Nation and a disconnected ring wire to rule out any interferance from my household wiring.