do you mean BT checked the line upto the house, they wont normally check the lines in the house as it's nothing to do with them once it get past the master socket?
First thing is to try a different ADSL filter on the socket the router is plugged into as the ones supplied by the ISP are always cheap nasty ones and do fail quite often, if that doesn't work and If it is possible to do so plug the router into the master socket and see if that improves things, if it does then it's your internal wiring thats at fault.
When you plug into the master socket, if you look at the socket there is two screws on the front, undo these and gently pull the faceplate towards you and it should come off and you will se another socket behind it, use this if possible as thats the test socket.
If it's still slow when plugged into the master socket get back onto BT and give them more grief.