You should reset your line and go through a new training period; this is the fastest way of getting back up to higher speeds.

Unplug all ADSL equipment, including filters for about 40 minutes, then plug them all back in and restart your PC, this should reset the line. Leave your PC connected permanently for the next 10 days, and download as many large files as you can, hi def radio streams, bittorrent, M$ service packs etc.

If your router is wireless, it is best to have the router next to the main socket and connect to your PC's wirelessly, this allows you to take off the cover of the master socket and use the "Test" socket underneath.

Switching from the front socket to the "Test" socket raised my internet speeds from 4Mbps to 6Mbps

Glad you're sorted, hopefully!

Yep, definitely makes sense to have the router as close to the main BT socket as possible, and use longer CAT-5 ethernet cables to your computers, if necessary. As has been said these can be pretty long (upto 100m theoretical, although you do sometimes notice a drop-off in performance when they're at 80+ m).

Steve